Index: Lib/test/test_posix.py
===================================================================
--- Lib/test/test_posix.py	(revision 82172)
+++ Lib/test/test_posix.py	(working copy)
@@ -12,8 +12,8 @@
 import shutil
 import unittest
 import warnings
+import sys
 
-
 warnings.filterwarnings('ignore', '.* potential security risk .*',
                         RuntimeWarning)
 
@@ -356,10 +356,59 @@
             finally:
                 os.chdir(curdir)
                 shutil.rmtree(base_path)
+        
+    def test_getgroups(self):
+        with os.popen('id -G') as idg:
+            groups = idg.read().strip()
+        if not groups:
+            raise unittest.SkipTest("need working 'id -G'")
+        grouplist = map(int, groups.split())
+        if sys.platform == "darwin" and len(grouplist) > 16:
+            raise unittest.SkipTest("grouplist cannot handle more"
+                                    " than 16 groups on darwin")
+        self.assertListEqual(grouplist, posix.getgroups())
 
+class PosixGroupsTester(unittest.TestCase):
+    
+    def setUp(self):
+        if posix.getuid() != 0:
+            raise unittest.SkipTest("not enough privileges")
+        if not hasattr(posix, 'getgroups'):
+            raise unittest.SkipTest("need posix.getgroups")
+        self.saved_groups = posix.getgroups()
 
+    def tearDown(self):
+        if hasattr(posix, 'setgroups'):
+            posix.setgroups(self.saved_groups)
+        elif hasattr(posix, 'initgroups'):
+            name = pwd.getpwuid(posix.getuid()).pw_name
+            posix.initgroups(name, self.saved_groups[0])
+
+    @unittest.skipUnless(hasattr(posix, 'initgroups'),
+                         "test needs posix.initgroups()")
+    def test_initgroups(self):
+        # find missing group
+        groups = sorted(self.saved_groups)
+        for g1,g2 in zip(groups[:-1], groups[1:]):
+            g = g1 + 1
+            if g < g2:
+                break
+        else:
+            g = g2 + 1
+        name = pwd.getpwuid(posix.getuid()).pw_name
+        posix.initgroups(name, g)
+        self.assertIn(g, posix.getgroups())
+        
+    @unittest.skipUnless(hasattr(posix, 'setgroups'),
+                         "test needs posix.setgroups()")
+    def test_setgroups(self):
+        for groups in [[0], range(16)]:
+            posix.setgroups(groups)
+            self.assertListEqual(groups, posix.getgroups())
+        
+
 def test_main():
-    test_support.run_unittest(PosixTester)
+    test_support.run_unittest(PosixTester, PosixGroupsTester)
 
 if __name__ == '__main__':
     test_main()
Index: Modules/posixmodule.c
===================================================================
--- Modules/posixmodule.c	(revision 82172)
+++ Modules/posixmodule.c	(working copy)
@@ -3842,6 +3842,10 @@
 
 
 #ifdef HAVE_GETGROUPS
+#  ifdef __APPLE__
+int _posix_getgroups(int, gid_t []) __asm("_getgroups");
+#    define getgroups _posix_getgroups
+#  endif
 PyDoc_STRVAR(posix_getgroups__doc__,
 "getgroups() -> list of group IDs\n\n\
 Return list of supplemental group IDs for the process.");
@@ -3860,7 +3864,7 @@
     gid_t grouplist[MAX_GROUPS];
     int n;
 
-    n = getgroups(MAX_GROUPS, grouplist);
+    n = _posix_getgroups(MAX_GROUPS, grouplist);
     if (n < 0)
         posix_error();
     else {