diff --git a/STL_Extension/include/CGAL/iterator.h b/STL_Extension/include/CGAL/iterator.h
index 33636da9d3a..6df0aa61497 100644
--- a/STL_Extension/include/CGAL/iterator.h
+++ b/STL_Extension/include/CGAL/iterator.h
@@ -485,6 +485,8 @@ template < class I, class P > struct Filter_iterator;
 
 template < class I, class P >
 bool operator==(const Filter_iterator<I,P>&, const Filter_iterator<I,P>&);
+template < class I, class P >
+bool operator<(const Filter_iterator<I,P>&, const Filter_iterator<I,P>&);
 
 template < class I, class P >
 struct Filter_iterator {
@@ -551,6 +553,7 @@ public:
   bool is_end() const { return (c_ == e_); }
 
   friend bool operator== <>(const Self&, const Self&);
+  friend bool operator< <>(const Self&, const Self&);
 };
 
 template < class I, class P >
@@ -572,6 +575,14 @@ bool operator==(const Filter_iterator<I,P>& it1,
   return it1.base() == it2.base();
 }
 
+template < class I, class P >
+inline
+bool operator<(const Filter_iterator<I,P>& it1,
+                const Filter_iterator<I,P>& it2)
+{
+  return it1.base() < it2.base();
+}
+
 template < class I, class P >
 inline
 bool operator!=(const Filter_iterator<I,P>& it1,