usernames and passwords

Hi .
I have a server running PostgreSQL Version 6.5, and i have a problem.
I grant some distinct privileges to some users (and that works). The
problem
is that those users can log in with any password. Each user has a
password
assigned but not matter the password the user use to log in, if the user
exists always will get access to the database.

Has anybony an idea?
Any response will be wellcome.

Thanks for all.


Vicent Vives.