diff --git a/lisp/Config.sun4c_41 b/lisp/Config.sun4c_41
index d64d467a1147ad910214e96292560db4c6b0ac08..60e5f09890285c7a308451b9f5f41b8af4cda927 100644
--- a/lisp/Config.sun4c_41
+++ b/lisp/Config.sun4c_41
@@ -1,4 +1,4 @@
-CPPFLAGS = -I. -I/usr/misc/.X11/include
+CPPFLAGS = -I. -I/usr/misc/.X11/include -DSUNOS
 CC = gcc # -Wall -Wstrict-prototypes -Wmissing-prototypes
 CPP = /lib/cpp
 CFLAGS = -g