Frank, I don't see how your transformer would disable JBoss AOP. The javadoc of java.lang.instrument.Instrumentation states that every registered transformer will be called. Specially, if you are not transforming the class... for what I understood, you are using the ClassFileTransformer only to register the class, and not to transform it, right? This seems to be a bug in the JVM. I would try a few things: - try to return the same bytecodes that you got as a parameter instead of null - try to run this using a different jvm - check to see if the bytecodes your ClassFileTransfomer is receiving are weaved or not, and also check the stack trace when it executes, to see if the classFileTransfomer is on the stack Let me know if any of this gives us a clue on what is wrong. View the original post : http://www.jboss.org/index.html?module=bb&op=viewtopic&p=4222676#... Reply to the post : http://www.jboss.org/index.html?module=bb&op=posting&mode=reply&a...