diff --git a/lisp/Config.pmax_mach b/lisp/Config.pmax_mach
index 4577db9dcb0d49b95a663b1e61be4f1106c10a8a..cd1282984f2b52a6dcc24571d48c08d1d4d5858b 100644
--- a/lisp/Config.pmax_mach
+++ b/lisp/Config.pmax_mach
@@ -1,4 +1,4 @@
-CPPFLAGS = -I. -I/usr/misc/.X11/include
+CPPFLAGS = -I. -I/usr/misc/.X11/include -I/usr/mach/include
 CC = gcc # -Wall -Wstrict-prototypes -Wmissing-prototypes
 CPP = /usr/cs/lib/cpp
 CFLAGS = -O