There's already a bug report open for this on Adobe's bug tracker, but I thought I'd ask here. I'm running OS X 10.5.4, and although I do have the Java 1.6 update installed, I've used the Java Preferences utility to make Java 1.5 the default runtime JVM. Flex Builder installs fine, but it crashes when I try to launch it (see bug reports here and here). My suspicion is that, somehow, the application launcher is still trying to use the 1.6 JVM and that's causing the crash. I'm trying to figure out if there's some fairly painless way to uninstall the Java 1.6 update so that there's no chance for Flex Builder to "find" it. Anyways. Does this sound familiar to anyone? I think it's probably similar to the problem that people have had running Eclipse with the Java 1.6 update on OS X.