diff --git a/testing/acceptance_test.py b/testing/acceptance_test.py
index 680afe133..0046d05b8 100644
--- a/testing/acceptance_test.py
+++ b/testing/acceptance_test.py
@@ -267,7 +267,7 @@ class TestGeneralUsage:
     def test_issue109_sibling_conftests_not_loaded(self, pytester: Pytester) -> None:
         sub1 = pytester.mkdir("sub1")
         sub2 = pytester.mkdir("sub2")
-        sub1.joinpath("conftest.py").write_text("assert 0")
+        sub1.joinpath("conftest.py").write_text("assert 0", encoding="utf-8")
         result = pytester.runpytest(sub2)
         assert result.ret == ExitCode.NO_TESTS_COLLECTED
         sub2.joinpath("__init__.py").touch()
@@ -467,7 +467,7 @@ class TestGeneralUsage:
         assert "invalid" in str(excinfo.value)
 
         p = pytester.path.joinpath("test_test_plugins_given_as_strings.py")
-        p.write_text("def test_foo(): pass")
+        p.write_text("def test_foo(): pass", encoding="utf-8")
         mod = types.ModuleType("myplugin")
         monkeypatch.setitem(sys.modules, "myplugin", mod)
         assert pytest.main(args=[str(pytester.path)], plugins=["myplugin"]) == 0
@@ -587,7 +587,7 @@ class TestInvocationVariants:
     def test_pyargs_importerror(self, pytester: Pytester, monkeypatch) -> None:
         monkeypatch.delenv("PYTHONDONTWRITEBYTECODE", False)
         path = pytester.mkpydir("tpkg")
-        path.joinpath("test_hello.py").write_text("raise ImportError")
+        path.joinpath("test_hello.py").write_text("raise ImportError", encoding="utf-8")
 
         result = pytester.runpytest("--pyargs", "tpkg.test_hello", syspathinsert=True)
         assert result.ret != 0
@@ -597,10 +597,10 @@ class TestInvocationVariants:
     def test_pyargs_only_imported_once(self, pytester: Pytester) -> None:
         pkg = pytester.mkpydir("foo")
         pkg.joinpath("test_foo.py").write_text(
-            "print('hello from test_foo')\ndef test(): pass"
+            "print('hello from test_foo')\ndef test(): pass", encoding="utf-8"
         )
         pkg.joinpath("conftest.py").write_text(
-            "def pytest_configure(config): print('configuring')"
+            "def pytest_configure(config): print('configuring')", encoding="utf-8"
         )
 
         result = pytester.runpytest(
diff --git a/testing/python/fixtures.py b/testing/python/fixtures.py
index d996f80bb..e62db8c26 100644
--- a/testing/python/fixtures.py
+++ b/testing/python/fixtures.py
@@ -287,7 +287,8 @@ class TestFillFixtures:
                 def spam():
                     return 'spam'
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         testfile = subdir.joinpath("test_spam.py")
         testfile.write_text(
@@ -296,7 +297,8 @@ class TestFillFixtures:
                 def test_spam(spam):
                     assert spam == "spam"
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         result = pytester.runpytest()
         result.stdout.fnmatch_lines(["*1 passed*"])
@@ -359,7 +361,8 @@ class TestFillFixtures:
                 def spam(request):
                     return request.param
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         testfile = subdir.joinpath("test_spam.py")
         testfile.write_text(
@@ -371,7 +374,8 @@ class TestFillFixtures:
                     assert spam == params['spam']
                     params['spam'] += 1
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         result = pytester.runpytest()
         result.stdout.fnmatch_lines(["*3 passed*"])
@@ -403,7 +407,8 @@ class TestFillFixtures:
                 def spam(request):
                     return request.param
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         testfile = subdir.joinpath("test_spam.py")
         testfile.write_text(
@@ -415,7 +420,8 @@ class TestFillFixtures:
                     assert spam == params['spam']
                     params['spam'] += 1
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         result = pytester.runpytest()
         result.stdout.fnmatch_lines(["*3 passed*"])
@@ -1037,10 +1043,11 @@ class TestRequestBasic:
                 def arg1():
                     pass
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         p = b.joinpath("test_module.py")
-        p.write_text("def test_func(arg1): pass")
+        p.write_text("def test_func(arg1): pass", encoding="utf-8")
         result = pytester.runpytest(p, "--fixtures")
         assert result.ret == 0
         result.stdout.fnmatch_lines(
@@ -1617,7 +1624,8 @@ class TestFixtureManagerParseFactories:
             def one():
                 return 1
             """
-            )
+            ),
+            encoding="utf-8",
         )
         package.joinpath("test_x.py").write_text(
             textwrap.dedent(
@@ -1625,7 +1633,8 @@ class TestFixtureManagerParseFactories:
                 def test_x(one):
                     assert one == 1
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         sub = package.joinpath("sub")
         sub.mkdir()
@@ -1638,7 +1647,8 @@ class TestFixtureManagerParseFactories:
                 def one():
                     return 2
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         sub.joinpath("test_y.py").write_text(
             textwrap.dedent(
@@ -1646,7 +1656,8 @@ class TestFixtureManagerParseFactories:
                 def test_x(one):
                     assert one == 2
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         reprec = pytester.inline_run()
         reprec.assertoutcome(passed=2)
@@ -1671,7 +1682,8 @@ class TestFixtureManagerParseFactories:
                 def teardown_module():
                     values[:] = []
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         package.joinpath("test_x.py").write_text(
             textwrap.dedent(
@@ -1680,7 +1692,8 @@ class TestFixtureManagerParseFactories:
                 def test_x():
                     assert values == ["package"]
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         package = pytester.mkdir("package2")
         package.joinpath("__init__.py").write_text(
@@ -1692,7 +1705,8 @@ class TestFixtureManagerParseFactories:
                 def teardown_module():
                     values[:] = []
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         package.joinpath("test_x.py").write_text(
             textwrap.dedent(
@@ -1701,7 +1715,8 @@ class TestFixtureManagerParseFactories:
                 def test_x():
                     assert values == ["package2"]
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         reprec = pytester.inline_run()
         reprec.assertoutcome(passed=2)
@@ -1714,7 +1729,7 @@ class TestFixtureManagerParseFactories:
         )
         pytester.syspathinsert(pytester.path.name)
         package = pytester.mkdir("package")
-        package.joinpath("__init__.py").write_text("")
+        package.joinpath("__init__.py").write_text("", encoding="utf-8")
         package.joinpath("conftest.py").write_text(
             textwrap.dedent(
                 """\
@@ -1731,7 +1746,8 @@ class TestFixtureManagerParseFactories:
                     yield values
                     values.pop()
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         package.joinpath("test_x.py").write_text(
             textwrap.dedent(
@@ -1742,7 +1758,8 @@ class TestFixtureManagerParseFactories:
                 def test_package(one):
                     assert values == ["package-auto", "package"]
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         reprec = pytester.inline_run()
         reprec.assertoutcome(passed=2)
@@ -1892,8 +1909,12 @@ class TestAutouseDiscovery:
         """
         )
         conftest.rename(a.joinpath(conftest.name))
-        a.joinpath("test_something.py").write_text("def test_func(): pass")
-        b.joinpath("test_otherthing.py").write_text("def test_func(): pass")
+        a.joinpath("test_something.py").write_text(
+            "def test_func(): pass", encoding="utf-8"
+        )
+        b.joinpath("test_otherthing.py").write_text(
+            "def test_func(): pass", encoding="utf-8"
+        )
         result = pytester.runpytest()
         result.stdout.fnmatch_lines(
             """
@@ -1939,7 +1960,8 @@ class TestAutouseManagement:
                     import sys
                     sys._myapp = "hello"
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         sub = pkgdir.joinpath("tests")
         sub.mkdir()
@@ -1952,7 +1974,8 @@ class TestAutouseManagement:
                 def test_app():
                     assert sys._myapp == "hello"
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         reprec = pytester.inline_run("-s")
         reprec.assertoutcome(passed=1)
@@ -2882,7 +2905,7 @@ class TestFixtureMarker:
             def browser(request):
 
                 def finalize():
-                    sys.stdout.write_text('Finalized')
+                    sys.stdout.write_text('Finalized', encoding='utf-8')
                 request.addfinalizer(finalize)
                 return {}
         """
@@ -2900,7 +2923,8 @@ class TestFixtureMarker:
                 def test_browser(browser):
                     assert browser['visited'] is True
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         reprec = pytester.runpytest("-s")
         for test in ["test_browser"]:
@@ -3855,7 +3879,8 @@ class TestParameterizedSubRequest:
                 def fix_with_param(request):
                     return request.param
                 """
-            )
+            ),
+            encoding="utf-8",
         )
 
         testfile = tests_dir.joinpath("test_foos.py")
@@ -3867,7 +3892,8 @@ class TestParameterizedSubRequest:
                 def test_foo(request):
                     request.getfixturevalue('fix_with_param')
                 """
-            )
+            ),
+            encoding="utf-8",
         )
 
         os.chdir(tests_dir)
@@ -4196,7 +4222,7 @@ class TestScopeOrdering:
                 └── test_2.py
         """
         root = pytester.mkdir("root")
-        root.joinpath("__init__.py").write_text("values = []")
+        root.joinpath("__init__.py").write_text("values = []", encoding="utf-8")
         sub1 = root.joinpath("sub1")
         sub1.mkdir()
         sub1.joinpath("__init__.py").touch()
@@ -4211,7 +4237,8 @@ class TestScopeOrdering:
                 yield values
                 assert values.pop() == "pre-sub1"
         """
-            )
+            ),
+            encoding="utf-8",
         )
         sub1.joinpath("test_1.py").write_text(
             textwrap.dedent(
@@ -4220,7 +4247,8 @@ class TestScopeOrdering:
             def test_1(fix):
                 assert values == ["pre-sub1"]
         """
-            )
+            ),
+            encoding="utf-8",
         )
         sub2 = root.joinpath("sub2")
         sub2.mkdir()
@@ -4236,7 +4264,8 @@ class TestScopeOrdering:
                 yield values
                 assert values.pop() == "pre-sub2"
         """
-            )
+            ),
+            encoding="utf-8",
         )
         sub2.joinpath("test_2.py").write_text(
             textwrap.dedent(
@@ -4245,7 +4274,8 @@ class TestScopeOrdering:
             def test_2(fix):
                 assert values == ["pre-sub2"]
         """
-            )
+            ),
+            encoding="utf-8",
         )
         reprec = pytester.inline_run()
         reprec.assertoutcome(passed=2)
diff --git a/testing/python/metafunc.py b/testing/python/metafunc.py
index c1cc9c3d3..a9e9b5269 100644
--- a/testing/python/metafunc.py
+++ b/testing/python/metafunc.py
@@ -1443,7 +1443,8 @@ class TestMetafuncFunctional:
                 def pytest_generate_tests(metafunc):
                     assert metafunc.function.__name__ == "test_1"
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         sub2.joinpath("conftest.py").write_text(
             textwrap.dedent(
@@ -1451,10 +1452,15 @@ class TestMetafuncFunctional:
                 def pytest_generate_tests(metafunc):
                     assert metafunc.function.__name__ == "test_2"
                 """
-            )
+            ),
+            encoding="utf-8",
+        )
+        sub1.joinpath("test_in_sub1.py").write_text(
+            "def test_1(): pass", encoding="utf-8"
+        )
+        sub2.joinpath("test_in_sub2.py").write_text(
+            "def test_2(): pass", encoding="utf-8"
         )
-        sub1.joinpath("test_in_sub1.py").write_text("def test_1(): pass")
-        sub2.joinpath("test_in_sub2.py").write_text("def test_2(): pass")
         result = pytester.runpytest("--keep-duplicates", "-v", "-s", sub1, sub2, sub1)
         result.assert_outcomes(passed=3)
 
diff --git a/testing/test_assertrewrite.py b/testing/test_assertrewrite.py
index 245241af2..057b609ac 100644
--- a/testing/test_assertrewrite.py
+++ b/testing/test_assertrewrite.py
@@ -160,7 +160,8 @@ class TestAssertionRewrite:
             "def special_asserter():\n"
             "    def special_assert(x, y):\n"
             "        assert x == y\n"
-            "    return special_assert\n"
+            "    return special_assert\n",
+            encoding="utf-8",
         )
         pytester.makeconftest('pytest_plugins = ["plugin"]')
         pytester.makepyfile("def test(special_asserter): special_asserter(1, 2)\n")
@@ -173,7 +174,9 @@ class TestAssertionRewrite:
         pytester.makepyfile(test_y="x = 1")
         xdir = pytester.mkdir("x")
         pytester.mkpydir(str(xdir.joinpath("test_Y")))
-        xdir.joinpath("test_Y").joinpath("__init__.py").write_text("x = 2")
+        xdir.joinpath("test_Y").joinpath("__init__.py").write_text(
+            "x = 2", encoding="utf-8"
+        )
         pytester.makepyfile(
             "import test_y\n"
             "import test_Y\n"
@@ -726,7 +729,7 @@ class TestAssertionRewrite:
 
 class TestRewriteOnImport:
     def test_pycache_is_a_file(self, pytester: Pytester) -> None:
-        pytester.path.joinpath("__pycache__").write_text("Hello")
+        pytester.path.joinpath("__pycache__").write_text("Hello", encoding="utf-8")
         pytester.makepyfile(
             """
             def test_rewritten():
@@ -903,7 +906,8 @@ def test_rewritten():
         pkg.joinpath("test_blah.py").write_text(
             """
 def test_rewritten():
-    assert "@py_builtins" in globals()"""
+    assert "@py_builtins" in globals()""",
+            encoding="utf-8",
         )
         assert pytester.runpytest().ret == 0
 
@@ -1066,7 +1070,7 @@ class TestAssertionRewriteHookDetails:
         source = tmp_path / "source.py"
         pyc = Path(str(source) + "c")
 
-        source.write_text("def test(): pass")
+        source.write_text("def test(): pass", encoding="utf-8")
         py_compile.compile(str(source), str(pyc))
 
         contents = pyc.read_bytes()
@@ -1092,7 +1096,7 @@ class TestAssertionRewriteHookDetails:
         fn = tmp_path / "source.py"
         pyc = Path(str(fn) + "c")
 
-        fn.write_text("def test(): assert True")
+        fn.write_text("def test(): assert True", encoding="utf-8")
 
         source_stat, co = _rewrite_test(fn, config)
         _write_pyc(state, co, source_stat, pyc)
@@ -1187,9 +1191,10 @@ class TestAssertionRewriteHookDetails:
                         data = pkgutil.get_data('foo.test_foo', 'data.txt')
                         assert data == b'Hey'
                 """
-            )
+            ),
+            encoding="utf-8",
         )
-        path.joinpath("data.txt").write_text("Hey")
+        path.joinpath("data.txt").write_text("Hey", encoding="utf-8")
         result = pytester.runpytest()
         result.stdout.fnmatch_lines(["*1 passed*"])
 
diff --git a/testing/test_collection.py b/testing/test_collection.py
index 302139872..3e1a04443 100644
--- a/testing/test_collection.py
+++ b/testing/test_collection.py
@@ -140,7 +140,7 @@ class TestCollectFS:
         ensure_file(tmp_path / ".bzr" / "test_notfound.py")
         ensure_file(tmp_path / "normal" / "test_found.py")
         for x in tmp_path.rglob("test_*.py"):
-            x.write_text("def test_hello(): pass", "utf-8")
+            x.write_text("def test_hello(): pass", encoding="utf-8")
 
         result = pytester.runpytest("--collect-only")
         s = result.stdout.str()
@@ -162,7 +162,7 @@ class TestCollectFS:
         bindir = "Scripts" if sys.platform.startswith("win") else "bin"
         ensure_file(pytester.path / "virtual" / bindir / fname)
         testfile = ensure_file(pytester.path / "virtual" / "test_invenv.py")
-        testfile.write_text("def test_hello(): pass")
+        testfile.write_text("def test_hello(): pass", encoding="utf-8")
 
         # by default, ignore tests inside a virtualenv
         result = pytester.runpytest()
@@ -192,7 +192,7 @@ class TestCollectFS:
         # norecursedirs takes priority
         ensure_file(pytester.path / ".virtual" / bindir / fname)
         testfile = ensure_file(pytester.path / ".virtual" / "test_invenv.py")
-        testfile.write_text("def test_hello(): pass")
+        testfile.write_text("def test_hello(): pass", encoding="utf-8")
         result = pytester.runpytest("--collect-in-virtualenv")
         result.stdout.no_fnmatch_line("*test_invenv*")
         # ...unless the virtualenv is explicitly given on the CLI
@@ -231,10 +231,14 @@ class TestCollectFS:
         )
         tmp_path = pytester.path
         ensure_file(tmp_path / "mydir" / "test_hello.py").write_text(
-            "def test_1(): pass"
+            "def test_1(): pass", encoding="utf-8"
+        )
+        ensure_file(tmp_path / "xyz123" / "test_2.py").write_text(
+            "def test_2(): 0/0", encoding="utf-8"
+        )
+        ensure_file(tmp_path / "xy" / "test_ok.py").write_text(
+            "def test_3(): pass", encoding="utf-8"
         )
-        ensure_file(tmp_path / "xyz123" / "test_2.py").write_text("def test_2(): 0/0")
-        ensure_file(tmp_path / "xy" / "test_ok.py").write_text("def test_3(): pass")
         rec = pytester.inline_run()
         rec.assertoutcome(passed=1)
         rec = pytester.inline_run("xyz123/test_2.py")
@@ -248,12 +252,14 @@ class TestCollectFS:
         """
         )
         tmp_path = pytester.path
-        ensure_file(tmp_path / "a" / "test_1.py").write_text("def test_a(): pass")
+        ensure_file(tmp_path / "a" / "test_1.py").write_text(
+            "def test_a(): pass", encoding="utf-8"
+        )
         ensure_file(tmp_path / "b" / "tests" / "test_2.py").write_text(
-            "def test_b(): pass"
+            "def test_b(): pass", encoding="utf-8"
         )
         ensure_file(tmp_path / "c" / "tests" / "test_3.py").write_text(
-            "def test_c(): pass"
+            "def test_c(): pass", encoding="utf-8"
         )
 
         # executing from rootdir only tests from `testpaths` directories
@@ -349,8 +355,8 @@ class TestCustomConftests:
         """
         )
         sub = pytester.mkdir("xy123")
-        ensure_file(sub / "test_hello.py").write_text("syntax error")
-        sub.joinpath("conftest.py").write_text("syntax error")
+        ensure_file(sub / "test_hello.py").write_text("syntax error", encoding="utf-8")
+        sub.joinpath("conftest.py").write_text("syntax error", encoding="utf-8")
         pytester.makepyfile("def test_hello(): pass")
         pytester.makepyfile(test_one="syntax error")
         result = pytester.runpytest("--fulltrace")
@@ -1060,13 +1066,18 @@ def test_fixture_scope_sibling_conftests(pytester: Pytester) -> None:
             def fix():
                 return 1
             """
-        )
+        ),
+        encoding="utf-8",
+    )
+    foo_path.joinpath("test_foo.py").write_text(
+        "def test_foo(fix): assert fix == 1", encoding="utf-8"
     )
-    foo_path.joinpath("test_foo.py").write_text("def test_foo(fix): assert fix == 1")
 
     # Tests in `food/` should not see the conftest fixture from `foo/`
     food_path = pytester.mkpydir("food")
-    food_path.joinpath("test_food.py").write_text("def test_food(fix): assert fix == 1")
+    food_path.joinpath("test_food.py").write_text(
+        "def test_food(fix): assert fix == 1", encoding="utf-8"
+    )
 
     res = pytester.runpytest()
     assert res.ret == 1
@@ -1197,7 +1208,8 @@ def test_collect_with_chdir_during_import(pytester: Pytester) -> None:
             os.chdir(%r)
             """
             % (str(subdir),)
-        )
+        ),
+        encoding="utf-8",
     )
     pytester.makepyfile(
         """
@@ -1227,8 +1239,12 @@ def test_collect_pyargs_with_testpaths(
 ) -> None:
     testmod = pytester.mkdir("testmod")
     # NOTE: __init__.py is not collected since it does not match python_files.
-    testmod.joinpath("__init__.py").write_text("def test_func(): pass")
-    testmod.joinpath("test_file.py").write_text("def test_func(): pass")
+    testmod.joinpath("__init__.py").write_text(
+        "def test_func(): pass", encoding="utf-8"
+    )
+    testmod.joinpath("test_file.py").write_text(
+        "def test_func(): pass", encoding="utf-8"
+    )
 
     root = pytester.mkdir("root")
     root.joinpath("pytest.ini").write_text(
@@ -1238,7 +1254,8 @@ def test_collect_pyargs_with_testpaths(
         addopts = --pyargs
         testpaths = testmod
     """
-        )
+        ),
+        encoding="utf-8",
     )
     monkeypatch.setenv("PYTHONPATH", str(pytester.path), prepend=os.pathsep)
     with monkeypatch.context() as mp:
@@ -1323,6 +1340,7 @@ def test_collect_symlink_out_of_tree(pytester: Pytester) -> None:
             assert request.node.nodeid == "test_real.py::test_nodeid"
         """
         ),
+        encoding="utf-8",
     )
 
     out_of_tree = pytester.mkdir("out_of_tree")
diff --git a/testing/test_config.py b/testing/test_config.py
index cdeb67ace..3aec5d763 100644
--- a/testing/test_config.py
+++ b/testing/test_config.py
@@ -87,7 +87,8 @@ class TestParseIni:
                 [pytest]
                 addopts = --verbose
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         config = pytester.parseconfig(tmp_path)
         assert config.option.color == "no"
@@ -127,7 +128,8 @@ class TestParseIni:
         """.format(
                     section=section
                 )
-            )
+            ),
+            encoding="utf-8",
         )
         config = pytester.parseconfig()
         assert config.getini("minversion") == "3.36"
@@ -150,7 +152,8 @@ class TestParseIni:
             [pytest]
             minversion = 2.0
         """
-            )
+            ),
+            encoding="utf-8",
         )
         pytester.path.joinpath("pytest.ini").write_text(
             textwrap.dedent(
@@ -158,13 +161,16 @@ class TestParseIni:
             [pytest]
             minversion = 1.5
         """
-            )
+            ),
+            encoding="utf-8",
         )
         config = pytester.parseconfigure(sub)
         assert config.getini("minversion") == "2.0"
 
     def test_ini_parse_error(self, pytester: Pytester) -> None:
-        pytester.path.joinpath("pytest.ini").write_text("addopts = -x")
+        pytester.path.joinpath("pytest.ini").write_text(
+            "addopts = -x", encoding="utf-8"
+        )
         result = pytester.runpytest()
         assert result.ret != 0
         result.stderr.fnmatch_lines("ERROR: *pytest.ini:1: no section header defined")
@@ -634,7 +640,7 @@ class TestConfigAPI:
     def test_getconftest_pathlist(self, pytester: Pytester, tmp_path: Path) -> None:
         somepath = tmp_path.joinpath("x", "y", "z")
         p = tmp_path.joinpath("conftest.py")
-        p.write_text(f"mylist = {['.', str(somepath)]}")
+        p.write_text(f"mylist = {['.', str(somepath)]}", encoding="utf-8")
         config = pytester.parseconfigure(p)
         assert (
             config._getconftest_pathlist("notexist", path=tmp_path, rootpath=tmp_path)
@@ -910,7 +916,8 @@ class TestConfigFromdictargs:
                 [pytest]
                 name = value
                 """
-            )
+            ),
+            encoding="utf-8",
         )
 
         inifilename = "../../foo/bar.ini"
@@ -927,7 +934,8 @@ class TestConfigFromdictargs:
                 name = wrong-value
                 should_not_be_set = true
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         with MonkeyPatch.context() as mp:
             mp.chdir(cwd)
@@ -1387,7 +1395,7 @@ class TestRootdir:
     )
     def test_with_ini(self, tmp_path: Path, name: str, contents: str) -> None:
         inipath = tmp_path / name
-        inipath.write_text(contents, "utf-8")
+        inipath.write_text(contents, encoding="utf-8")
 
         a = tmp_path / "a"
         a.mkdir()
@@ -1446,7 +1454,7 @@ class TestRootdir:
     ) -> None:
         p = tmp_path / name
         p.touch()
-        p.write_text(contents, "utf-8")
+        p.write_text(contents, encoding="utf-8")
         rootpath, inipath, ini_config = determine_setup(str(p), [str(tmp_path)])
         assert rootpath == tmp_path
         assert inipath == p
@@ -1542,7 +1550,8 @@ class TestOverrideIniArgs:
             custom = 1.0""".format(
                     section=section
                 )
-            )
+            ),
+            encoding="utf-8",
         )
         pytester.makeconftest(
             """
diff --git a/testing/test_conftest.py b/testing/test_conftest.py
index f857cde04..7ec9feb8b 100644
--- a/testing/test_conftest.py
+++ b/testing/test_conftest.py
@@ -47,8 +47,12 @@ class TestConftestValueAccessGlobal:
     ) -> Generator[Path, None, None]:
         tmp_path = tmp_path_factory.mktemp("basedir", numbered=True)
         tmp_path.joinpath("adir/b").mkdir(parents=True)
-        tmp_path.joinpath("adir/conftest.py").write_text("a=1 ; Directory = 3")
-        tmp_path.joinpath("adir/b/conftest.py").write_text("b=2 ; a = 1.5")
+        tmp_path.joinpath("adir/conftest.py").write_text(
+            "a=1 ; Directory = 3", encoding="utf-8"
+        )
+        tmp_path.joinpath("adir/b/conftest.py").write_text(
+            "b=2 ; a = 1.5", encoding="utf-8"
+        )
         if request.param == "inpackage":
             tmp_path.joinpath("adir/__init__.py").touch()
             tmp_path.joinpath("adir/b/__init__.py").touch()
@@ -123,8 +127,12 @@ class TestConftestValueAccessGlobal:
 
 def test_conftest_in_nonpkg_with_init(tmp_path: Path, _sys_snapshot) -> None:
     tmp_path.joinpath("adir-1.0/b").mkdir(parents=True)
-    tmp_path.joinpath("adir-1.0/conftest.py").write_text("a=1 ; Directory = 3")
-    tmp_path.joinpath("adir-1.0/b/conftest.py").write_text("b=2 ; a = 1.5")
+    tmp_path.joinpath("adir-1.0/conftest.py").write_text(
+        "a=1 ; Directory = 3", encoding="utf-8"
+    )
+    tmp_path.joinpath("adir-1.0/b/conftest.py").write_text(
+        "b=2 ; a = 1.5", encoding="utf-8"
+    )
     tmp_path.joinpath("adir-1.0/b/__init__.py").touch()
     tmp_path.joinpath("adir-1.0/__init__.py").touch()
     ConftestWithSetinitial(tmp_path.joinpath("adir-1.0", "b"))
@@ -167,7 +175,7 @@ def test_conftest_global_import(pytester: Pytester) -> None:
         sub = Path("sub")
         sub.mkdir()
         subconf = sub / "conftest.py"
-        subconf.write_text("y=4")
+        subconf.write_text("y=4", encoding="utf-8")
         mod2 = conf._importconftest(subconf, importmode="prepend", rootpath=Path.cwd())
         assert mod != mod2
         assert mod2.y == 4
@@ -246,7 +254,8 @@ def test_conftest_confcutdir(pytester: Pytester) -> None:
             def pytest_addoption(parser):
                 parser.addoption("--xyz", action="store_true")
             """
-        )
+        ),
+        encoding="utf-8",
     )
     result = pytester.runpytest("-h", "--confcutdir=%s" % x, x)
     result.stdout.fnmatch_lines(["*--xyz*"])
@@ -274,9 +283,12 @@ def test_installed_conftest_is_picked_up(pytester: Pytester, tmp_path: Path) ->
             @pytest.fixture
             def fix(): return None
             """
-        )
+        ),
+        encoding="utf-8",
+    )
+    tmp_path.joinpath("foo", "test_it.py").write_text(
+        "def test_it(fix): pass", encoding="utf-8"
     )
-    tmp_path.joinpath("foo", "test_it.py").write_text("def test_it(fix): pass")
     result = pytester.runpytest("--pyargs", "foo")
     assert result.ret == 0
 
@@ -401,7 +413,8 @@ def test_conftest_existing_junitxml(pytester: Pytester) -> None:
             def pytest_addoption(parser):
                 parser.addoption("--xyz", action="store_true")
             """
-        )
+        ),
+        encoding="utf-8",
     )
     pytester.makefile(ext=".xml", junit="")  # Writes junit.xml
     result = pytester.runpytest("-h", "--junitxml", "junit.xml")
@@ -412,7 +425,7 @@ def test_conftest_import_order(pytester: Pytester, monkeypatch: MonkeyPatch) ->
     ct1 = pytester.makeconftest("")
     sub = pytester.mkdir("sub")
     ct2 = sub / "conftest.py"
-    ct2.write_text("")
+    ct2.write_text("", encoding="utf-8")
 
     def impct(p, importmode, root):
         return p
@@ -450,7 +463,8 @@ def test_fixture_dependency(pytester: Pytester) -> None:
             def bar(foo):
                 return 'bar'
             """
-        )
+        ),
+        encoding="utf-8",
     )
     subsub = sub.joinpath("subsub")
     subsub.mkdir()
@@ -467,7 +481,8 @@ def test_fixture_dependency(pytester: Pytester) -> None:
             def test_event_fixture(bar):
                 assert bar == 'sub bar'
             """
-        )
+        ),
+        encoding="utf-8",
     )
     result = pytester.runpytest("sub")
     result.stdout.fnmatch_lines(["*1 passed*"])
@@ -481,10 +496,11 @@ def test_conftest_found_with_double_dash(pytester: Pytester) -> None:
             def pytest_addoption(parser):
                 parser.addoption("--hello-world", action="store_true")
             """
-        )
+        ),
+        encoding="utf-8",
     )
     p = sub.joinpath("test_hello.py")
-    p.write_text("def test_hello(): pass")
+    p.write_text("def test_hello(): pass", encoding="utf-8")
     result = pytester.runpytest(str(p) + "::test_hello", "-h")
     result.stdout.fnmatch_lines(
         """
@@ -508,7 +524,8 @@ class TestConftestVisibility:
                 def fxtr():
                     return "from-package"
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         package.joinpath("test_pkgroot.py").write_text(
             textwrap.dedent(
@@ -516,7 +533,8 @@ class TestConftestVisibility:
                 def test_pkgroot(fxtr):
                     assert fxtr == "from-package"
                 """
-            )
+            ),
+            encoding="utf-8",
         )
 
         swc = package.joinpath("swc")
@@ -530,7 +548,8 @@ class TestConftestVisibility:
                 def fxtr():
                     return "from-swc"
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         swc.joinpath("test_with_conftest.py").write_text(
             textwrap.dedent(
@@ -538,7 +557,8 @@ class TestConftestVisibility:
                 def test_with_conftest(fxtr):
                     assert fxtr == "from-swc"
                 """
-            )
+            ),
+            encoding="utf-8",
         )
 
         snc = package.joinpath("snc")
@@ -551,7 +571,8 @@ class TestConftestVisibility:
                     assert fxtr == "from-package"   # No local conftest.py, so should
                                                     # use value from parent dir's
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         print("created directory structure:")
         for x in pytester.path.glob("**/"):
@@ -625,7 +646,8 @@ def test_search_conftest_up_to_inifile(
             @pytest.fixture
             def fix1(): pass
             """
-        )
+        ),
+        encoding="utf-8",
     )
     src.joinpath("test_foo.py").write_text(
         textwrap.dedent(
@@ -635,7 +657,8 @@ def test_search_conftest_up_to_inifile(
             def test_2(out_of_reach):
                 pass
             """
-        )
+        ),
+        encoding="utf-8",
     )
     root.joinpath("conftest.py").write_text(
         textwrap.dedent(
@@ -644,7 +667,8 @@ def test_search_conftest_up_to_inifile(
             @pytest.fixture
             def out_of_reach(): pass
             """
-        )
+        ),
+        encoding="utf-8",
     )
 
     args = [str(src)]
@@ -727,7 +751,8 @@ def test_required_option_help(pytester: Pytester) -> None:
             def pytest_addoption(parser):
                 parser.addoption("--xyz", action="store_true", required=True)
             """
-        )
+        ),
+        encoding="utf-8",
     )
     result = pytester.runpytest("-h", x)
     result.stdout.no_fnmatch_line("*argument --xyz is required*")
diff --git a/testing/test_doctest.py b/testing/test_doctest.py
index d2944fa2b..dfe569987 100644
--- a/testing/test_doctest.py
+++ b/testing/test_doctest.py
@@ -357,7 +357,8 @@ class TestDoctests:
                         >>> 1/0
                         '''
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         result = pytester.runpytest("--doctest-modules")
         result.stdout.fnmatch_lines(
@@ -448,7 +449,8 @@ class TestDoctests:
                 """\
                 import asdalsdkjaslkdjasd
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         pytester.maketxtfile(
             """
@@ -492,7 +494,8 @@ class TestDoctests:
                         2
                     '''
                 """
-            )
+            ),
+            encoding="utf-8",
         )
         result = pytester.runpytest(p, "--doctest-modules")
         result.stdout.fnmatch_lines(
@@ -1566,7 +1569,9 @@ def test_warning_on_unwrap_of_broken_object(
 
 def test_is_setup_py_not_named_setup_py(tmp_path: Path) -> None:
     not_setup_py = tmp_path.joinpath("not_setup.py")
-    not_setup_py.write_text('from setuptools import setup; setup(name="foo")')
+    not_setup_py.write_text(
+        'from setuptools import setup; setup(name="foo")', encoding="utf-8"
+    )
     assert not _is_setup_py(not_setup_py)
 
 
diff --git a/testing/test_monkeypatch.py b/testing/test_monkeypatch.py
index 500e24453..8a9dd600b 100644
--- a/testing/test_monkeypatch.py
+++ b/testing/test_monkeypatch.py
@@ -324,7 +324,8 @@ def test_importerror(pytester: Pytester) -> None:
 
         x = 1
     """
-        )
+        ),
+        encoding="utf-8",
     )
     pytester.path.joinpath("test_importerror.py").write_text(
         textwrap.dedent(
@@ -332,7 +333,8 @@ def test_importerror(pytester: Pytester) -> None:
         def test_importerror(monkeypatch):
             monkeypatch.setattr('package.a.x', 2)
     """
-        )
+        ),
+        encoding="utf-8",
     )
     result = pytester.runpytest()
     result.stdout.fnmatch_lines(
@@ -434,11 +436,13 @@ def test_syspath_prepend_with_namespace_packages(
         ns = d.joinpath("ns_pkg")
         ns.mkdir()
         ns.joinpath("__init__.py").write_text(
-            "__import__('pkg_resources').declare_namespace(__name__)"
+            "__import__('pkg_resources').declare_namespace(__name__)", encoding="utf-8"
         )
         lib = ns.joinpath(dirname)
         lib.mkdir()
-        lib.joinpath("__init__.py").write_text("def check(): return %r" % dirname)
+        lib.joinpath("__init__.py").write_text(
+            "def check(): return %r" % dirname, encoding="utf-8"
+        )
 
     monkeypatch.syspath_prepend("hello")
     import ns_pkg.hello
diff --git a/testing/test_pathlib.py b/testing/test_pathlib.py
index 481d7a606..0fd372b51 100644
--- a/testing/test_pathlib.py
+++ b/testing/test_pathlib.py
@@ -100,13 +100,13 @@ class TestImportPath:
     def setuptestfs(self, path: Path) -> None:
         # print "setting up test fs for", repr(path)
         samplefile = path / "samplefile"
-        samplefile.write_text("samplefile\n")
+        samplefile.write_text("samplefile\n", encoding="utf-8")
 
         execfile = path / "execfile"
-        execfile.write_text("x=42")
+        execfile.write_text("x=42", encoding="utf-8")
 
         execfilepy = path / "execfile.py"
-        execfilepy.write_text("x=42")
+        execfilepy.write_text("x=42", encoding="utf-8")
 
         d = {1: 2, "hello": "world", "answer": 42}
         path.joinpath("samplepickle").write_bytes(pickle.dumps(d, 1))
diff --git a/tox.ini b/tox.ini
index 88ae16dea..05c1842af 100644
--- a/tox.ini
+++ b/tox.ini
@@ -38,6 +38,10 @@ passenv =
 setenv =
     _PYTEST_TOX_DEFAULT_POSARGS={env:_PYTEST_TOX_POSARGS_DOCTESTING:} {env:_PYTEST_TOX_POSARGS_LSOF:} {env:_PYTEST_TOX_POSARGS_XDIST:}
 
+    # See https://docs.python.org/3/library/io.html#io-encoding-warning
+    # If we don't enable this, neither can any of our downstream users!
+    PYTHONWARNDEFAULTENCODING=1
+
     # Configuration to run with coverage similar to CI, e.g.
     # "tox -e py37-coverage".
     coverage: _PYTEST_TOX_COVERAGE_RUN=coverage run -m