You can not do it in a general case. If you happen to know you have a directory-based CLASSPATH (not url as in applets or whatever), you can do a file search. You have to look thru each entry in java.class.path and search in there. If the entry is a jar, you will have to crack it open to look. I wrote a command-line program to search for classes, you are welcome to try it. http://www.avitek.com/landers/findclass.java.txt In the more general case, you will have a ClassLoader that will not tell you (thru the ClassLoader API) where it is getting classes from. So there is no way to do a search without knowing the class names.