diff -r 40ad90580935 Lib/ntpath.py
--- a/Lib/ntpath.py	Thu Jun 02 23:40:24 2011 -0700
+++ b/Lib/ntpath.py	Fri Jun 03 15:53:42 2011 -0500
@@ -672,3 +672,16 @@
 def sameopenfile(f1, f2):
     """Test whether two file objects reference the same file"""
     return _getfileinformation(f1) == _getfileinformation(f2)
+
+
+try:
+    # The genericpath.isdir implementation uses os.stat and checks the mode
+    # attribute to tell whether or not the path is a directory.
+    # This is overkill on Windows - just pass the path to FindFirstFile and
+    # check the attribute from there.
+    from nt import _isdir
+except ImportError:
+    from genericpath import isdir as _isdir
+
+def isdir(path):
+    return _isdir(path)
diff -r 40ad90580935 Modules/posixmodule.c
--- a/Modules/posixmodule.c	Thu Jun 02 23:40:24 2011 -0700
+++ b/Modules/posixmodule.c	Fri Jun 03 15:53:42 2011 -0500
@@ -2819,6 +2819,42 @@
                                 info.nFileIndexHigh,
                                 info.nFileIndexLow);
 }
+
+static PyObject *
+posix__isdir(PyObject *self, PyObject *args)
+{
+    PyObject *opath;
+    char *path;
+    PyUnicodeObject *po;
+    DWORD attributes;
+
+    if (PyArg_ParseTuple(args, "U|:_isdir", &po)) {
+        Py_UNICODE *wpath = PyUnicode_AS_UNICODE(po);
+
+        attributes = GetFileAttributesW(wpath);
+        if (attributes == INVALID_FILE_ATTRIBUTES)
+            Py_RETURN_FALSE;
+        goto check;
+    }
+    /* Drop the argument parsing error as narrow strings
+       are also valid. */
+    PyErr_Clear();
+
+    if (!PyArg_ParseTuple(args, "O&:_isdir",
+                          PyUnicode_FSConverter, &opath))
+        return NULL;
+
+    path = PyBytes_AsString(opath);
+    attributes = GetFileAttributesA(path);
+    if (attributes == INVALID_FILE_ATTRIBUTES)
+        Py_RETURN_FALSE;
+
+check:
+    if (attributes & FILE_ATTRIBUTE_DIRECTORY)
+        Py_RETURN_TRUE;
+    else
+        Py_RETURN_FALSE;
+}
 #endif /* MS_WINDOWS */
 
 PyDoc_STRVAR(posix_mkdir__doc__,
@@ -8055,6 +8091,7 @@
     {"_getfullpathname",        posix__getfullpathname, METH_VARARGS, NULL},
     {"_getfinalpathname",       posix__getfinalpathname, METH_VARARGS, NULL},
     {"_getfileinformation",     posix__getfileinformation, METH_VARARGS, NULL},
+    {"_isdir",                  posix__isdir, METH_VARARGS, NULL},
 #endif
 #ifdef HAVE_GETLOADAVG
     {"getloadavg",      posix_getloadavg, METH_NOARGS, posix_getloadavg__doc__},