Index: Lib/test/test_posix.py
===================================================================
--- Lib/test/test_posix.py	(revision 73420)
+++ Lib/test/test_posix.py	(working copy)
@@ -175,10 +175,15 @@
             posix.chdir(os.curdir)
             self.assertRaises(OSError, posix.chdir, test_support.TESTFN)
 
-    def test_lsdir(self):
-        if hasattr(posix, 'lsdir'):
-            self.assert_(test_support.TESTFN in posix.lsdir(os.curdir))
+    def test_listdir(self):
+        if hasattr(posix, 'listdir'):
+            self.assert_(test_support.TESTFN in posix.listdir(os.curdir))
 
+    def test_listdir_default(self):
+        # When listdir is called without argument, it's the same as listdir(os.curdir)
+        if hasattr(posix, 'listdir'):
+            self.assert_(test_support.TESTFN in posix.listdir())
+    
     def test_access(self):
         if hasattr(posix, 'access'):
             self.assert_(posix.access(test_support.TESTFN, os.R_OK))
Index: Modules/posixmodule.c
===================================================================
--- Modules/posixmodule.c	(revision 73420)
+++ Modules/posixmodule.c	(working copy)
@@ -2190,8 +2190,13 @@
 	/* If on wide-character-capable OS see if argument
 	   is Unicode and if so use wide API.  */
 	if (unicode_file_names()) {
-		PyObject *po;
-		if (PyArg_ParseTuple(args, "U:listdir", &po)) {
+		PyObject *po = NULL;
+		if (PyArg_ParseTuple(args, "|U:listdir", &po)) {
+			if (po == NULL) { // Default arg: "."
+				po = PyUnicode_FromWideChar(L".", 1 * sizeof(wchar_t));
+			} else {
+				Py_INCREF(po);
+			}
 			WIN32_FIND_DATAW wFileData;
 			Py_UNICODE *wnamebuf;
 			/* Overallocate for \\*.*\0 */
@@ -2202,6 +2207,7 @@
 				return NULL;
 			}
 			wcscpy(wnamebuf, PyUnicode_AS_UNICODE(po));
+			Py_DECREF(po);
 			if (len > 0) {
 				Py_UNICODE wch = wnamebuf[len-1];
 				if (wch != L'/' && wch != L'\\' && wch != L':')
@@ -2409,12 +2415,16 @@
 	int arg_is_unicode = 1;
 
 	errno = 0;
-	if (!PyArg_ParseTuple(args, "U:listdir", &v)) {
+	if (!PyArg_ParseTuple(args, "|U:listdir", &v)) {
 		arg_is_unicode = 0;
 		PyErr_Clear();
 	}
-	if (!PyArg_ParseTuple(args, "et:listdir", Py_FileSystemDefaultEncoding, &name))
+	if (!PyArg_ParseTuple(args, "|et:listdir", Py_FileSystemDefaultEncoding, &name))
 		return NULL;
+	if (name == NULL) { // Default arg: "."
+        name = (char *)PyMem_Malloc(2); // "." + \0
+        strcpy(name, ".");
+	}
 	if ((dirp = opendir(name)) == NULL) {
 		return posix_error_with_allocated_filename(name);
 	}