I actually have a workable solution, but I still don't understand why this is needed. In my EAR file I included the files to support the newer version of TreeCache (erby, jgroups, jboss-common-core 2.2.8, jcip-annotations, c3p0, and jbosscache-core 2.2.0). With these files in my EAR file, and referred to from a manifest of my WAR file, the MDB would not deploy with the error mentioned below. To fix this, I moved the files above into my WAR's lib directory (and my code that directly referenced it) and removed it from the manifest file. Now, everything starts up and runs correctly. I'm guessing that one of the JAR files for TreeCache is doing something to the JNDI lookup for StdJMSPool, but I can't figure out what it would be doing that would cause a problem. I did briefly try upgrading to JBoss 4.2.3, and things appeared to start correctly, but I had other issues I didn't want to fix right now (particularly JSF) View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4203264#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...