In jboss-messaging-client.jar there are several class files from the JMX package such as org.jboss.mx.server.ServerConfig. These appear to be from an old release of the jboss JMX as they refer to classes that no longer exist in the jBoss application server (such as JBossMXServerConfig ), so if they are used by the class loader then you get a class not found exception. We get this if we try to use jboss messaging in an application that's running in jboss application server 4.04GA and exposing beans to the application server's mBeanServer. If we switch to another messaging server (ActiveMQ) and remove the jboss-messaging-client.jar then it all works fine. Is there a reason that these classes are in the client jar, are they needed or could they be updated to be compaitable with the latest application server? View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=3978428#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...