bad version number

I have tomcat 6 and newly installed updated jdk. My old apps from another machine and Tomcat 5 / earlier jdk work fine. But when I recompile a java servlet .... the app won't deploy.

Can someone tell me what's going on from the error log?

I can see that it says

java.lang.UnsupportedClassVersionError: Bad version number in .class file

What the heck to I do to fix this? Here's the full error log:

http://geocities.com/rogerfgay/other/catalina.2007-02-15.log

[498 byte] By [RogerFGay1a] at [2007-11-26 18:35:20]