The startGroovy.bat file supplied with Groovy 1.5.6 does not pass the System CLASSPATH variable onto the GroovyStarter class. The Unix startGroovy does the following - if [ -n "$CP" ] ; then CP="$CP":. elif [ -n "$CLASSPATH" ] ; then CP="$CLASSPATH":. else CP=. fi The Windows startGroovy.bat does - if "x" == "x%CP%" goto empty_cp :non_empty_cp set CP=%CP%;. goto after_cp :empty_cp set CP=. :after_c Note , the CLASSPATH variable is ignored. I think this issue is the result of the fix for issue GROOVY-2644. In 1.5.5 the equivalent bit of script did the following - if "x" == "x%CP%" goto empty_cp :non_empty_cp set CP=%CP%;. goto after_cp :empty_cp set CP=. if "x" == "x%CLASSPATH%" goto after_cp set STARTER_CLASSPATH=%STARTER_CLASSPATH%;%CLASSPATH% :after_cp So, the CLASSPATH in 1.5.5 was passed on via the STARTER_CLASSPATH to the Java executable. Suggest that the correct script should be - if "x" == "x%CP%" goto empty_cp :non_empty_cp set CP=%CP%;. goto after_cp :empty_cp set CP=. if "x" == "x%CLASSPATH%" goto after_cp set CP=%CLASSPATH%;%CP% :after_cp