diff -r 6d998a43102b Lib/importlib/_bootstrap.py
--- a/Lib/importlib/_bootstrap.py	Thu Aug 29 14:37:47 2013 +0100
+++ b/Lib/importlib/_bootstrap.py	Thu Aug 29 13:57:14 2013 -0400
@@ -1369,63 +1369,64 @@
         self._loaders = loaders
         # Base (directory) path
         self.path = path or '.'
-        self._path_mtime = -1
-        self._path_cache = set()
-        self._relaxed_path_cache = set()
 
     def invalidate_caches(self):
         """Invalidate the directory mtime."""
-        self._path_mtime = -1
+        self._path_mtime[self.path] = -1
 
     find_module = _find_module_shim
 
+    _path_mtime = {}
+    _path_cache = {}
+    _relaxed_path_cache = {}
+
     def find_loader(self, fullname):
         """Try to find a loader for the specified module, or the namespace
         package portions. Returns (loader, list-of-portions)."""
         is_namespace = False
         tail_module = fullname.rpartition('.')[2]
-        try:
-            mtime = _os.stat(self.path).st_mtime
-        except OSError:
-            mtime = -1
-        if mtime != self._path_mtime:
-            self._fill_cache()
-            self._path_mtime = mtime
+        self._fill_cache()
         # tail_module keeps the original casing, for __file__ and friends
         if _relax_case():
-            cache = self._relaxed_path_cache
+            cache = self._relaxed_path_cache[self.path]
             cache_module = tail_module.lower()
         else:
-            cache = self._path_cache
+            cache = self._path_cache[self.path]
             cache_module = tail_module
         # Check if the module is the name of a directory (and thus a package).
         if cache_module in cache:
             base_path = _path_join(self.path, tail_module)
-            if _path_isdir(base_path):
-                for suffix, loader in self._loaders:
-                    init_filename = '__init__' + suffix
-                    full_path = _path_join(base_path, init_filename)
-                    if _path_isfile(full_path):
-                        return (loader(fullname, full_path), [base_path])
-                else:
-                    # A namespace package, return the path if we don't also
-                    #  find a module in the next section.
-                    is_namespace = True
+            self._fill_cache(base_path)
+            for suffix, loader in self._loaders:
+                init_filename = '__init__' + suffix
+                full_path = _path_join(base_path, init_filename)
+                if init_filename in self._path_cache[base_path]:
+                    return loader(fullname, full_path), [base_path]
+            else:
+                # A namespace package, return the path if we don't also
+                #  find a module in the next section.
+                is_namespace = True
         # Check for a file w/ a proper suffix exists.
         for suffix, loader in self._loaders:
             full_path = _path_join(self.path, tail_module + suffix)
             _verbose_message('trying {}'.format(full_path), verbosity=2)
             if cache_module + suffix in cache:
-                if _path_isfile(full_path):
-                    return (loader(fullname, full_path), [])
+                return loader(fullname, full_path), []
         if is_namespace:
             _verbose_message('possible namespace for {}'.format(base_path))
-            return (None, [base_path])
-        return (None, [])
+            return None, [base_path]
+        return None, []
 
-    def _fill_cache(self):
+    def _fill_cache(self, path=None):
         """Fill the cache of potential modules and packages for this directory."""
-        path = self.path
+        path = path if path is not None else self.path
+        try:
+            mtime = _os.stat(path).st_mtime
+        except OSError:
+            mtime = -1
+        if mtime == self._path_mtime.get(path, 0):
+            return
+        self._path_mtime[path] = mtime
         try:
             contents = _os.listdir(path)
         except (FileNotFoundError, PermissionError, NotADirectoryError):
@@ -1435,7 +1436,7 @@
         # We store two cached versions, to handle runtime changes of the
         # PYTHONCASEOK environment variable.
         if not sys.platform.startswith('win'):
-            self._path_cache = set(contents)
+            self._path_cache[path] = set(contents)
         else:
             # Windows users can import modules with case-insensitive file
             # suffixes (for legacy reasons). Make the suffix lowercase here
@@ -1450,9 +1451,9 @@
                 else:
                     new_name = name
                 lower_suffix_contents.add(new_name)
-            self._path_cache = lower_suffix_contents
+            self._path_cache[path] = lower_suffix_contents
         if sys.platform.startswith(_CASE_INSENSITIVE_PLATFORMS):
-            self._relaxed_path_cache = {fn.lower() for fn in contents}
+            self._relaxed_path_cache[path] = {fn.lower() for fn in contents}
 
     @classmethod
     def path_hook(cls, *loader_details):