diff --git a/hw/xquartz/X11Application.m b/hw/xquartz/X11Application.m
index 988a86bae..a5159b6a8 100644
--- a/hw/xquartz/X11Application.m
+++ b/hw/xquartz/X11Application.m
@@ -50,6 +50,8 @@
 #include <unistd.h>
 #include <AvailabilityMacros.h>
 
+#include <pthread.h>
+
 #include <Xplugin.h>
 
 // pbproxy/pbproxy.h