How do you save, how do you re-open (exactly - i.e. what menu options do you click etc)? Have you checked that the Java source files for each of your classes exists on disk (i.e. look using Finder in your scenario's folder) after saving and after trying to re-open? Can you send the debug log both after saving and closing Greenfoot, and after attempting to re-open? (see the support page for details on how to find the debug log file).