diff --git a/make.h b/make.h
index 9515490..0405087 100644
--- a/make.h
+++ b/make.h
@@ -59,7 +59,9 @@
 
 #ifdef	HAVE_UNISTD_H
 #include <unistd.h>
-#ifdef	_POSIX_VERSION
+/* Ultrix's unistd.h always defines _POSIX_VERSION, but you only get
+   POSIX.1 behavior with `cc -YPOSIX', which predefines POSIX itself!  */
+#if defined (_POSIX_VERSION) && !defined (ultrix)
 #define	POSIX
 #endif
 #endif