In particular, the section on Windows (16.6.3.2, the version at http://docs.python.org/library/multiprocessing.html#windows, if there happen to be others). It mentions that the 'main module' must be able to be 'safely imported', but doesn't make it clear that it needs the .py extension, so that it can be imported at all. Just thought it might help others where I spent a few minutes trying to work out what was going wrong.