diff -r 61d9c561b63d .idea/dictionaries/emptysquare.xml
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/.idea/dictionaries/emptysquare.xml	Thu Jul 25 13:51:59 2013 -0400
@@ -0,0 +1,7 @@
+<component name="ProjectDictionaryState">
+  <dictionary name="emptysquare">
+    <words>
+      <w>ceval</w>
+    </words>
+  </dictionary>
+</component>
\ No newline at end of file
diff -r 61d9c561b63d Lib/test/test_threading.py
--- a/Lib/test/test_threading.py	Thu Jul 25 12:11:55 2013 -0400
+++ b/Lib/test/test_threading.py	Thu Jul 25 13:51:59 2013 -0400
@@ -444,6 +444,27 @@
         self.assertEqual(out, b'')
         self.assertEqual(err, b'')
 
+    @unittest.skipUnless(hasattr(os, 'fork'), "needs os.fork()")
+    def test_is_alive_after_fork(self):
+        # Try hard to trigger #18418: is_alive() could sometimes be True on
+        # threads that vanished after a fork.
+
+        old_interval = sys.getswitchinterval()
+        self.addCleanup(sys.setswitchinterval, old_interval)
+
+        # Make the bug more likely to manifest.
+        sys.setswitchinterval(1e-6)
+
+        for i in range(20):
+            t = threading.Thread(target=lambda: None)
+            t.start()
+            pid = os.fork()
+            if pid == 0:
+                os._exit(1 if t.is_alive() else 0)
+            else:
+                (_, exit_status) = os.waitpid(pid, 0)
+                self.assertEqual(0, exit_status)
+
 
 class ThreadJoinOnShutdown(BaseTestCase):
 
diff -r 61d9c561b63d Lib/threading.py
--- a/Lib/threading.py	Thu Jul 25 12:11:55 2013 -0400
+++ b/Lib/threading.py	Thu Jul 25 13:51:59 2013 -0400
@@ -1,5 +1,6 @@
 """Thread module emulating a subset of Java's threading model."""
 
+import itertools
 import sys as _sys
 import _thread
 
@@ -931,11 +932,13 @@
     global _active_limbo_lock
     _active_limbo_lock = _allocate_lock()
 
-    # fork() only copied the current thread; clear references to others.
+    # fork() only copied the current thread; clear references to others,
+    # including any that were interrupted before moving themselves from
+    # _limbo to _active.
     new_active = {}
     current = current_thread()
     with _active_limbo_lock:
-        for thread in _active.values():
+        for thread in itertools.chain(_active.values(), _limbo.values()):
             # Any lock/condition variable may be currently locked or in an
             # invalid state, so we reinitialize them.
             thread._reset_internal_locks()