diff --git a/Fltk/SpherePosition_Widget.h b/Fltk/SpherePosition_Widget.h
index 2187eefe582f02c4c2455d8fa8b0852fa71f9634..11f1700afe2cd2d9e7986b3a2ed456b595c9105f 100644
--- a/Fltk/SpherePosition_Widget.h
+++ b/Fltk/SpherePosition_Widget.h
@@ -20,6 +20,7 @@
 // 
 // Please report all bugs and problems to <gmsh@geuz.org>.
 
+#include <math.h>
 #include <FL/Fl_Widget.H>
 
 // A small 2D widget to set the coordinates of a point on the unit