diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index 0036845..fac37b0 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -136,6 +136,15 @@
    Availability: Unix.
 
 
+.. function:: initgroups(username, gid)
+
+   Call the system initgroups() to initialize the group access list with all of
+   the groups of which the specified username is a member, plus the specified
+   group id. Availability: Unix.
+
+   .. versionadded:: 2.7
+
+
 .. function:: getlogin()
 
    Return the name of the user logged in on the controlling terminal of the
diff --git a/Lib/test/test_posix.py b/Lib/test/test_posix.py
index aaa3841..c6da157 100644
--- a/Lib/test/test_posix.py
+++ b/Lib/test/test_posix.py
@@ -5,6 +5,7 @@
 # Skip these tests if there is no posix module.
 posix = test_support.import_module('posix')
 
+import errno
 import time
 import os
 import pwd
@@ -83,6 +84,27 @@
                 new_group_ids = (current_group_ids[0]+1, -1, -1)
                 self.assertRaises(OSError, posix.setresgid, *new_group_ids)
 
+    @unittest.skipUnless(hasattr(posix, 'initgroups'),
+                         "test needs os.initgroups()")
+    def test_initgroups(self):
+        # It takes a string and an integer; check that it raises a TypeError
+        # for other argument lists.
+        self.assertRaises(TypeError, posix.initgroups)
+        self.assertRaises(TypeError, posix.initgroups, None)
+        self.assertRaises(TypeError, posix.initgroups, 3, "foo")
+        self.assertRaises(TypeError, posix.initgroups, "foo", 3, object())
+
+        # If a non-privileged user invokes it, it should fail with OSError
+        # EPERM.
+        if os.getuid() != 0:
+            name = pwd.getpwuid(posix.getuid()).pw_name
+            try:
+                posix.initgroups(name, 13)
+            except OSError as e:
+                self.assertEquals(e.errno, errno.EPERM)
+            else:
+                self.fail("Expected OSError to be raised by initgroups")
+
     def test_statvfs(self):
         if hasattr(posix, 'statvfs'):
             self.assertTrue(posix.statvfs(os.curdir))
diff --git a/Misc/NEWS b/Misc/NEWS
index a8b56e2..92f9026 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -490,6 +490,10 @@
 Library
 -------
 
+- Issue #7333: The `posix` module gains an `initgroups()` function providing
+  access to the initgroups(3) C library call on Unix systems which implement
+  it.  Patch by Jean-Paul Calderone.
+
 - Issue #7408: Fixed distutils.tests.sdist so it doesn't check for group
   ownership when the group is not forced, because the group may be different
   from the user's group and inherit from its container when the test is run.
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 68e7286..8956e93 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -3861,6 +3861,30 @@
 }
 #endif
 
+#ifdef HAVE_INITGROUPS
+PyDoc_STRVAR(posix_initgroups__doc__,
+"initgroups(username, gid) -> None\n\n\
+Call the system initgroups() to initialize the group access list with all of\n\
+the groups of which the specified username is a member, plus the specified\n\
+group id.");
+
+static PyObject *
+posix_initgroups(PyObject *self, PyObject *args)
+{
+	char *username;
+	long gid;
+
+	if (!PyArg_ParseTuple(args, "sl:initgroups", &username, &gid))
+		return NULL;
+
+	if (initgroups(username, (gid_t) gid) == -1)
+		return PyErr_SetFromErrno(PyExc_OSError);
+
+	Py_INCREF(Py_None);
+	return Py_None;
+}
+#endif
+
 #ifdef HAVE_GETPGID
 PyDoc_STRVAR(posix_getpgid__doc__,
 "getpgid(pid) -> pgid\n\n\
@@ -8629,6 +8653,9 @@
 #ifdef HAVE_SETGROUPS
 	{"setgroups",	posix_setgroups, METH_O, posix_setgroups__doc__},
 #endif /* HAVE_SETGROUPS */
+#ifdef HAVE_INITGROUPS
+	{"initgroups",	posix_initgroups, METH_VARARGS, posix_initgroups__doc__},
+#endif /* HAVE_INITGROUPS */
 #ifdef HAVE_GETPGID
 	{"getpgid",	posix_getpgid, METH_VARARGS, posix_getpgid__doc__},
 #endif /* HAVE_GETPGID */
diff --git a/configure b/configure
index 55e9534..8391a17 100755
--- a/configure
+++ b/configure
@@ -1,5 +1,5 @@
 #! /bin/sh
-# From configure.in Revision: 76558 .
+# From configure.in Revision: 76568 .
 # Guess values for system-dependent variables and create Makefiles.
 # Generated by GNU Autoconf 2.61 for python 2.7.
 #
@@ -17895,11 +17895,12 @@
 
 
 
+
 for ac_func in alarm setitimer getitimer bind_textdomain_codeset chown \
  clock confstr ctermid execv fchmod fchown fork fpathconf ftime ftruncate \
  gai_strerror getgroups getlogin getloadavg getpeername getpgid getpid \
  getpriority getresuid getresgid getpwent getspnam getspent getsid getwd \
- kill killpg lchmod lchown lstat mkfifo mknod mktime \
+ initgroups kill killpg lchmod lchown lstat mkfifo mknod mktime \
  mremap nice pathconf pause plock poll pthread_init \
  putenv readlink realpath \
  select sem_open sem_timedwait sem_getvalue sem_unlink setegid seteuid \
diff --git a/configure.in b/configure.in
index 95d100a..e7e6b91 100644
--- a/configure.in
+++ b/configure.in
@@ -2547,7 +2547,7 @@
  clock confstr ctermid execv fchmod fchown fork fpathconf ftime ftruncate \
  gai_strerror getgroups getlogin getloadavg getpeername getpgid getpid \
  getpriority getresuid getresgid getpwent getspnam getspent getsid getwd \
- kill killpg lchmod lchown lstat mkfifo mknod mktime \
+ initgroups kill killpg lchmod lchown lstat mkfifo mknod mktime \
  mremap nice pathconf pause plock poll pthread_init \
  putenv readlink realpath \
  select sem_open sem_timedwait sem_getvalue sem_unlink setegid seteuid \
diff --git a/pyconfig.h.in b/pyconfig.h.in
index 7541213..fd58443 100644
--- a/pyconfig.h.in
+++ b/pyconfig.h.in
@@ -301,6 +301,9 @@
 /* Define to 1 if you have the `getpeername' function. */
 #undef HAVE_GETPEERNAME
 
+/* Define to 1 if you have the `initgroups' function. */
+#undef HAVE_INITGROUPS
+
 /* Define to 1 if you have the `getpgid' function. */
 #undef HAVE_GETPGID