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