java -jar fails for working Netbeans project

[mistertransistor]

I have a Netbeans project that uses the RXTX serial port library to read data. It works fine in the IDE. However if I try to launch it under DOS using
java -jar myProg.jar
the application starts OK, but when I try to use the serial port I get a java.lang.NoClassDefFound error for gnu/io/SerialPortEventListener

I'm not a java expert but I assume this is a classpath problem. I followed the instructions for installing RXTX and obviously it works under NetBeans. This involved installing a dll and a jar file in appropriate directories.

Can anyone tell me how to get the stand-alone execution working?

Thanks
Andrew