diff --git a/Lib/unittest/loader.py b/Lib/unittest/loader.py
--- a/Lib/unittest/loader.py
+++ b/Lib/unittest/loader.py
@@ -35,6 +35,14 @@
         return testFailure
 
 
+def _ispackage(path):
+    """Guess whether a path refers to a package directory."""
+    if os.path.isdir(path):
+        for ext in ('.py', '.pyc'):
+            if os.path.isfile(os.path.join(path, '__init__' + ext)):
+                return True
+    return False
+
 def _make_failed_import_test(name, suiteClass):
     message = 'Failed to import test module: %s\n%s' % (
         name, traceback.format_exc())
@@ -283,7 +291,7 @@
         if os.path.isdir(os.path.abspath(start_dir)):
             start_dir = os.path.abspath(start_dir)
             if start_dir != top_level_dir:
-                is_not_importable = not os.path.isfile(os.path.join(start_dir, '__init__.py'))
+                is_not_importable = not _ispackage(start_dir)
         else:
             # support for discovery from dotted module names
             try:
@@ -451,8 +459,7 @@
                         msg % (mod_name, module_dir, expected_dir))
                 return self.loadTestsFromModule(module, pattern=pattern), False
         elif os.path.isdir(full_path):
-            if (not namespace and
-                not os.path.isfile(os.path.join(full_path, '__init__.py'))):
+            if not namespace and not _ispackage(full_path):
                 return None, False
 
             load_tests = None
diff --git a/Lib/unittest/test/test_discovery.py b/Lib/unittest/test/test_discovery.py
--- a/Lib/unittest/test/test_discovery.py
+++ b/Lib/unittest/test/test_discovery.py
@@ -414,22 +414,6 @@
         # invoked when the start_dir is a package (and not the top level).
         # http://bugs.python.org/issue22457
 
-        # Test data: we expect the following:
-        # an isfile to verify the package, then importing and scanning
-        # as per _find_tests' normal behaviour.
-        # We expect to see our load_tests hook called once.
-        vfs = {abspath('/toplevel'): ['startdir'],
-               abspath('/toplevel/startdir'): ['__init__.py']}
-        def list_dir(path):
-            return list(vfs[path])
-        self.addCleanup(setattr, os, 'listdir', os.listdir)
-        os.listdir = list_dir
-        self.addCleanup(setattr, os.path, 'isfile', os.path.isfile)
-        os.path.isfile = lambda path: path.endswith('.py')
-        self.addCleanup(setattr, os.path, 'isdir', os.path.isdir)
-        os.path.isdir = lambda path: not path.endswith('.py')
-        self.addCleanup(sys.path.remove, abspath('/toplevel'))
-
         class Module(object):
             paths = []
             load_tests_args = []
@@ -443,16 +427,29 @@
             def __eq__(self, other):
                 return self.path == other.path
 
-        loader = unittest.TestLoader()
-        loader._get_module_from_name = lambda name: Module(name)
-        loader.suiteClass = lambda thing: thing
+        # Test data: we expect the following:
+        # an isfile to verify the package, then importing and scanning
+        # as per _find_tests' normal behaviour.
+        # We expect to see our load_tests hook called once.
+        self.addCleanup(setattr, os.path, 'isfile', os.path.isfile)
+        self.addCleanup(setattr, os.path, 'isdir', os.path.isdir)
+        self.addCleanup(sys.path.remove, abspath('/toplevel'))
+        for ext in ('.py', '.pyc'):
+            with self.subTest(ext=ext):
+                os.path.isfile = lambda path: path.endswith('__init__' + ext)
+                os.path.isdir = lambda path: not os.path.isfile(path)
 
-        suite = loader.discover('/toplevel/startdir', top_level_dir='/toplevel')
+                loader = unittest.TestLoader()
+                loader._get_module_from_name = lambda name: Module(name)
+                loader.suiteClass = lambda thing: thing
 
-        # We should have loaded tests from the package __init__.
-        # (normally this would be nested TestSuites.)
-        self.assertEqual(suite,
-                         [['load_tests called startdir']])
+                suite = loader.discover('/toplevel/startdir',
+                                        top_level_dir='/toplevel')
+
+                # We should have loaded tests from the package __init__.
+                # (normally this would be nested TestSuites.)
+                self.assertEqual(suite,
+                                 [['load_tests called startdir']])
 
     def setup_import_issue_tests(self, fakefile):
         listdir = os.listdir