"Could not initialize class"?

I have a Servlet application running on Tomcat 6. I've got the

following error when I load a JSP file:

org.apache.jasper.JasperException: javax.servlet.ServletException:

java.lang.NoClassDefFoundError: Could not initialize class

org.apache.struts.taglib.bean.MessageTag

What would possibly cause this error?

Thanks!

[372 byte] By [yunga] at [2007-11-26 20:42:47]