diff --git a/libtransmission/rpcimpl.c b/libtransmission/rpcimpl.c
index 19cfabdfc..98b629d04 100644
--- a/libtransmission/rpcimpl.c
+++ b/libtransmission/rpcimpl.c
@@ -10,6 +10,10 @@
  * $Id$
  */
 
+#if defined( HAVE_ZLIB ) && ( _FILE_OFFSET_BITS == 64 )
+ #define _LARGEFILE64_SOURCE
+#endif
+
 #include <assert.h>
 #include <ctype.h> /* isdigit */
 #include <errno.h>