diff --git a/_pytest/assertion/rewrite.py b/_pytest/assertion/rewrite.py
index 50d8062ae..aa33f1352 100644
--- a/_pytest/assertion/rewrite.py
+++ b/_pytest/assertion/rewrite.py
@@ -163,9 +163,9 @@ class AssertionRewritingHook(object):
                     self.session = session
                     del session
         else:
-            for marked in self._must_rewrite:
-                if marked.startswith(name):
-                    return True
+            toplevel_name = name.split('.', 1)[0]
+            if toplevel_name in self._must_rewrite:
+                return True
         return False
 
     def mark_rewrite(self, *names):
diff --git a/_pytest/pytester.py b/_pytest/pytester.py
index fc5b3ebd9..7831655d1 100644
--- a/_pytest/pytester.py
+++ b/_pytest/pytester.py
@@ -16,6 +16,7 @@ from _pytest._code import Source
 import py
 import pytest
 from _pytest.main import Session, EXIT_OK
+from _pytest.assertion.rewrite import AssertionRewritingHook
 
 
 def pytest_addoption(parser):
@@ -685,8 +686,17 @@ class Testdir:
            ``pytest.main()`` instance should use.
 
         :return: A :py:class:`HookRecorder` instance.
-
         """
+        # When running py.test inline any plugins active in the main
+        # test process are already imported.  So this disables the
+        # warning which will trigger to say they can no longer be
+        # re-written, which is fine as they are already re-written.
+        orig_warn = AssertionRewritingHook._warn_already_imported
+        def revert():
+            AssertionRewritingHook._warn_already_imported = orig_warn
+        self.request.addfinalizer(revert)
+        AssertionRewritingHook._warn_already_imported = lambda *a: None
+
         rec = []
         class Collect:
             def pytest_configure(x, config):