--- Imath/ImathMatrix.h
+++ Imath/ImathMatrix.h
@@ -37,6 +37,8 @@
 #ifndef INCLUDED_IMATHMATRIX_H
 #define INCLUDED_IMATHMATRIX_H
 
+#include <cstring>
+
 //----------------------------------------------------------------
 //
 //      2D (3x3) and 3D (4x4) transformation matrix templates.