diff --git a/src/bhtree.c b/src/bhtree.c
index 1db701a..cb16afb 100644
--- a/src/bhtree.c
+++ b/src/bhtree.c
@@ -38,6 +38,7 @@
 */
 #include <stdio.h>
 #include <math.h>
+#include <stdlib.h>
 #include "bhtree.h"
 
 /* Barnes - Hut Trees ? */