diff --git a/include/osg/Shader b/include/osg/Shader
index 4347cf9a5..6b7ad0bca 100644
--- a/include/osg/Shader
+++ b/include/osg/Shader
@@ -306,6 +306,7 @@ class OSG_EXPORT Shader : public osg::Object
         /** osg::Programs that this osg::Shader is attached to */
         typedef std::set< osg::Program* > ProgramSet;
         ProgramSet                      _programSet;
+        OpenThreads::Mutex _programSetMutex;
         mutable osg::buffered_value< osg::ref_ptr<ShaderObjects> > _pcsList;
 
     private:
diff --git a/src/osg/Shader.cpp b/src/osg/Shader.cpp
index 2b8481e42..de9055707 100644
--- a/src/osg/Shader.cpp
+++ b/src/osg/Shader.cpp
@@ -483,6 +483,7 @@ Shader::PerContextShader* Shader::getPCS(osg::State& state) const
 
 bool Shader::addProgramRef( Program* program )
 {
+    OpenThreads::ScopedLock<OpenThreads::Mutex> lk(_programSetMutex);
     ProgramSet::iterator itr = _programSet.find(program);
     if( itr != _programSet.end() ) return false;
 
@@ -492,6 +493,7 @@ bool Shader::addProgramRef( Program* program )
 
 bool Shader::removeProgramRef( Program* program )
 {
+    OpenThreads::ScopedLock<OpenThreads::Mutex> lk(_programSetMutex);
     ProgramSet::iterator itr = _programSet.find(program);
     if( itr == _programSet.end() ) return false;
 
@@ -507,6 +509,7 @@ void Shader::dirtyShader()
         if( _pcsList[cxt].valid() ) _pcsList[cxt]->requestCompile();
     }
 
+    OpenThreads::ScopedLock<OpenThreads::Mutex> lk(_programSetMutex);
     // Also mark Programs that depend on us as needing relink.
     for( ProgramSet::iterator itr = _programSet.begin();
         itr != _programSet.end(); ++itr )