diff --git a/libobjc/ChangeLog b/libobjc/ChangeLog
index 910ae986922..50413c8328e 100644
--- a/libobjc/ChangeLog
+++ b/libobjc/ChangeLog
@@ -1,3 +1,8 @@
+2006-02-21  Rainer Orth  <ro@TechFak.Uni-Bielefeld.DE>
+
+	PR libobjc/26309
+	* thr-objc.c (_XOPEN_SOURCE): Don't define on Tru64 UNIX.
+
 2006-01-24  David Ayers  <d.ayers@inode.at>
 
 	PR libobjc/9751
diff --git a/libobjc/thr-objc.c b/libobjc/thr-objc.c
index a859e1eb0fb..7d6375aaa55 100644
--- a/libobjc/thr-objc.c
+++ b/libobjc/thr-objc.c
@@ -1,5 +1,6 @@
 /* GNU Objective C Runtime Thread Interface.
-   Copyright (C) 1999 Free Software Foundation, Inc.
+   Copyright (C) 1999, 2000, 2001, 2002, 2003, 2005, 2006
+   Free Software Foundation, Inc.
 
 This file is part of GCC.
 
@@ -26,8 +27,11 @@ Boston, MA 02110-1301, USA.  */
 #define _LIBOBJC
 /* The line below is needed for declarations of functions such as
    pthread_mutexattr_settype, without which gthr-posix.h may fail to
-   compile within libobjc.  */
+   compile within libobjc.  Unfortunately, this breaks compilation on
+   Tru64 UNIX V4.0F, so disable it there.  */
+#ifndef __osf__
 #define _XOPEN_SOURCE 500
+#endif
 #include "config.h"
 #include "tconfig.h"
 #include "coretypes.h"