diff -r 330c7aa2922b Lib/test/test_threading.py
--- a/Lib/test/test_threading.py	Tue Jul 09 19:15:43 2013 +0200
+++ b/Lib/test/test_threading.py	Mon Jul 15 20:04:36 2013 -0400
@@ -444,6 +444,30 @@
         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()
+        try:
+            # Make the bug more likely to manifest.
+            sys.setswitchinterval(1e-6)
+
+            for i in range(1, 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,
+                        "#18418 triggered after %d trials" % i)
+        finally:
+            sys.setswitchinterval(old_interval)
+
 
 class ThreadJoinOnShutdown(BaseTestCase):
 
diff -r 330c7aa2922b Lib/threading.py
--- a/Lib/threading.py	Tue Jul 09 19:15:43 2013 +0200
+++ b/Lib/threading.py	Mon Jul 15 20:04:36 2013 -0400
@@ -623,10 +623,13 @@
     def _bootstrap_inner(self):
         try:
             self._set_ident()
-            self._started.set()
+            # Issue #18418: Order matters, since another thread can fork at any
+            # time.  Add self to _active, so we'll be marked dead after a fork,
+            # then set __started.  Thus is_alive() must be False after a fork.
             with _active_limbo_lock:
                 _active[self._ident] = self
                 del _limbo[self]
+            self._started.set()
 
             if _trace_hook:
                 _sys.settrace(_trace_hook)