Postgis error

Hi,

Using postgreSQL 8.4 and Postgis 1.4 in an Ubuntu 10.04 Server, when
trying to create a table:
CREATE TABLE observation (
(...)
spatial_value geometry,
);

I get this error message:
ERROR: type "geometry" does not exist

I supposed that installing postgis I would get the geographical object,
so I am not sure if I have to do anything else.

Thanks in advance for your help,
Alvaro.