diff --git a/src/osgText/Font.cpp b/src/osgText/Font.cpp
index 1a3e17607..b9d413f20 100644
--- a/src/osgText/Font.cpp
+++ b/src/osgText/Font.cpp
@@ -492,6 +492,8 @@ void Font::addGlyph(const FontResolution& fontRes, unsigned int charcode, Glyph*
 
 void Font::assignGlyphToGlyphTexture(Glyph* glyph, ShaderTechnique shaderTechnique)
 {
+    OpenThreads::ScopedLock<OpenThreads::Mutex> lock(_glyphMapMutex);
+
     int posX=0,posY=0;
 
     GlyphTexture* glyphTexture = 0;