diff --git a/src/osgPlugins/osgParticle/IO_ParticleSystemUpdater.cpp b/src/osgPlugins/osgParticle/IO_ParticleSystemUpdater.cpp
index 5c2b9456e..cb8faaa34 100644
--- a/src/osgPlugins/osgParticle/IO_ParticleSystemUpdater.cpp
+++ b/src/osgPlugins/osgParticle/IO_ParticleSystemUpdater.cpp
@@ -37,7 +37,7 @@ bool PSU_writeLocalData(const osg::Object &obj, osgDB::Output &fw)
 {
     const osgParticle::ParticleSystemUpdater &myobj = static_cast<const osgParticle::ParticleSystemUpdater &>(obj);    
 
-    for (int i=0; i<myobj.getNumParticleSystems(); ++i) {
+    for (unsigned int i=0; i<myobj.getNumParticleSystems(); ++i) {
         fw.writeObject(*myobj.getParticleSystem(i));
     }