Weird import problem

[Anton81]

I have a directory structure like

NS/dir1/file1.py
NS/dir2/file2.py

if in the python shell I type

import NS.dir1.file1

it works, however typing

import NS.dir2.file2

fails with

ImportError: No module named dir2.file2

Any ideas what could go wrong?
Directory permissions seem to be OK.