>Now how can I get the second class to know that the parent class Abc exists?

I see you've gotten the SET PROC answer, but just for your information, VFP7 has a new clause for DEFINE CLASS:

DEFINE CLASS ClassName1 AS ParentClass [OF ClassLibrary] [OLEPUBLIC]

which will be able to do what you want. For now, the class has to be avialable when you run the thing, either with SET PROC or SET CLASSLIB