Using an ALIAS in WHERE clause

I'm using a query with similar functionality to the following:

SELECT id,
sum(hours) AS totalhours
FROM mytable
WHERE totalhours > 50;

I get the following error:
Attribute 'totalhours' not found.

Am I not allowed to use an alias here? If not, how can I get my desired
output?

Thanks

Note: I'm using postgresql 7.2

--
Ron St.Pierre
Syscor R&D
tel: 250-361-1681
email: rstpierre(at)syscor(dot)com