diff --git a/src/mpp/mpp.c b/src/mpp/mpp.c
index b2bdc3f6d1..3aae6b125e 100644
--- a/src/mpp/mpp.c
+++ b/src/mpp/mpp.c
@@ -2,6 +2,7 @@
  *	(C) Copyright 10/17/86 by Carnegie Mellon University
  */
 #include <stdio.h>
+#include <unistd.h>
 
 extern char *malloc();