diff --git a/libguile/scmsigs.c b/libguile/scmsigs.c
index 86fce0f..e062f24 100644
--- a/libguile/scmsigs.c
+++ b/libguile/scmsigs.c
@@ -1,4 +1,4 @@
-/* Copyright (C) 1995,1996,1997,1998,1999,2000,2001, 2002, 2004, 2006, 2007, 2008, 2009, 2011 Free Software Foundation, Inc.
+/* Copyright (C) 1995,1996,1997,1998,1999,2000,2001, 2002, 2004, 2006, 2007, 2008, 2009, 2011, 2012 Free Software Foundation, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public License
@@ -183,6 +183,12 @@ read_signal_pipe_data (void * data)
   return NULL;
 }
   
+/* Lock to protect async queuing.  Since `system-async-mark' conses, and
+   thus acquires the GC lock, this lock ensures that the signal thread
+   doesn't call `system-async-mark' in the middle of a `fork'.  */
+static scm_i_pthread_mutex_t signal_async_mutex =
+  SCM_I_PTHREAD_MUTEX_INITIALIZER;
+
 static SCM
 signal_delivery_thread (void *data)
 {
@@ -212,7 +218,11 @@ signal_delivery_thread (void *data)
 	  h = SCM_SIMPLE_VECTOR_REF (signal_handler_asyncs, sig);
 	  t = SCM_SIMPLE_VECTOR_REF (signal_handler_threads, sig);
 	  if (scm_is_true (h))
+	    {
+	      scm_i_pthread_mutex_lock (&signal_async_mutex);
 	    scm_system_async_mark_for_thread (h, t);
+	      scm_i_pthread_mutex_unlock (&signal_async_mutex);
+	    }
 	}
       else if (sigdata.n == 0)
 	break; /* the signal pipe was closed. */
@@ -713,6 +723,8 @@ scm_init_scmsigs ()
   signal_handler_asyncs = scm_c_make_vector (NSIG, SCM_BOOL_F);
   signal_handler_threads = scm_c_make_vector (NSIG, SCM_BOOL_F);
 
+  scm_c_atfork_lock_static_mutex (&signal_async_mutex);
+
   for (i = 0; i < NSIG; i++)
     {
 #ifdef HAVE_SIGACTION