diff --git a/OPTYap/pthread_locks.h b/OPTYap/pthread_locks.h
index 8539a9f18..2fdc22ec5 100644
--- a/OPTYap/pthread_locks.h
+++ b/OPTYap/pthread_locks.h
@@ -15,6 +15,8 @@
 *									 *
 *************************************************************************/
 
+#include <pthread.h>
+
 /* ----------------------------------- **
 **      Atomic locks for PTHREADS      **
 ** ----------------------------------- */