diff --git a/include/X11/Xos.h b/include/X11/Xos.h
index 28dfc673a..37d268f27 100644
--- a/include/X11/Xos.h
+++ b/include/X11/Xos.h
@@ -80,6 +80,7 @@ in this Software without prior written authorization from The Open Group.
 #   include <unistd.h>
 #  endif
 #  ifdef WIN32
+#   include <io.h> // For open()
 #   include <X11/Xw32defs.h>
 #  else
 #   include <sys/file.h>