Python 2.5 zlib trouble

[Jesramz]

Python 2.5.6 (r256:88840, Sep 22 2011, 13:45:58)
[GCC 4.5.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import zlib
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named zlib
>>>

But if I run Python2.7 I get:

Python 2.7.1+ (r271:86832, Apr 11 2011, 18:05:24)
[GCC 4.5.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import zlib
>>>

It seems to work.