--- a/Modules/posixmodule.c	
+++ a/Modules/posixmodule.c	
@@ -164,6 +164,15 @@ corresponding Unix manual entries for more information on calls.");
 
 #ifndef _MSC_VER
 
+#ifdef WITH_THREAD
+typedef void *PyThread_type_lock;
+
+extern PyThread_type_lock PyThread_allocate_lock(void);
+extern int PyThread_acquire_lock(PyThread_type_lock, int);
+extern void PyThread_release_lock(PyThread_type_lock);
+PyThread_type_lock keymutex;
+#endif
+
 #if defined(__sgi)&&_COMPILER_VERSION>=700
 /* declare ctermid_r if compiling with MIPSPro 7.x in ANSI C mode
    (default) */
@@ -350,7 +359,6 @@ extern int lstat(const char *, struct stat *);
 #endif
 #endif
 
-
 #ifndef MS_WINDOWS
 PyObject *
 _PyInt_FromUid(uid_t uid)
@@ -3796,7 +3804,15 @@ posix_fork1(PyObject *self, PyObject *noargs)
     pid_t pid;
     int result = 0;
     _PyImport_AcquireLock();
+#ifdef WITH_THREAD
+    if (keymutex == NULL)
+        keymutex = PyThread_allocate_lock();
+    PyThread_acquire_lock(keymutex, 1);
+#endif
     pid = fork1();
+#ifdef WITH_THREAD
+    PyThread_release_lock(keymutex);
+#endif
     if (pid == 0) {
         /* child: this clobbers and resets the import lock. */
         PyOS_AfterFork();
@@ -3829,7 +3845,15 @@ posix_fork(PyObject *self, PyObject *noargs)
     pid_t pid;
     int result = 0;
     _PyImport_AcquireLock();
+#ifdef WITH_THREAD
+    if (keymutex == NULL)
+        keymutex = PyThread_allocate_lock();
+    PyThread_acquire_lock(keymutex, 1);
+#endif
     pid = fork();
+#ifdef WITH_THREAD
+    PyThread_release_lock(keymutex);
+#endif
     if (pid == 0) {
         /* child: this clobbers and resets the import lock. */
         PyOS_AfterFork();
@@ -3955,7 +3979,15 @@ posix_forkpty(PyObject *self, PyObject *noargs)
     pid_t pid;
 
     _PyImport_AcquireLock();
+#ifdef WITH_THREAD
+    if (keymutex == NULL)
+        keymutex = PyThread_allocate_lock();
+    PyThread_acquire_lock(keymutex, 1);
+#endif
     pid = forkpty(&master_fd, NULL, NULL, NULL);
+#ifdef WITH_THREAD
+    PyThread_release_lock(keymutex);
+#endif
     if (pid == 0) {
         /* child: this clobbers and resets the import lock. */
         PyOS_AfterFork();
--- a/Python/thread.c	
+++ a/Python/thread.c	
@@ -233,7 +233,7 @@ struct key {
 };
 
 static struct key *keyhead = NULL;
-static PyThread_type_lock keymutex = NULL;
+PyThread_type_lock keymutex = NULL;
 static int nkeys = 0;  /* PyThread_create_key() hands out nkeys+1 next */
 
 /* Internal helper.