diff --git a/sbt.sh b/sbt.sh
index acdcf92..0382ba9 100755
--- a/sbt.sh
+++ b/sbt.sh
@@ -5,5 +5,8 @@
 # @see http://www.scala-sbt.org/0.13/tutorial/Manual-Installation.html
 #
 
-SBT_OPTS="-Xms512M -Xmx1536M -Xss1M -XX:+CMSClassUnloadingEnabled -XX:MaxPermSize=256M"
+if [ -z "$SBT_OPTS" ]; then
+    SBT_OPTS="-Xms512M -Xmx1536M -Xss1M -XX:+CMSClassUnloadingEnabled -XX:MaxPermSize=256M"
+fi  
+
 exec  java $SBT_OPTS -jar `dirname $0`/sbt-launch.jar "$@"