ERROR: column 'xxx' does not exist (under v. 7.4.1)

Using psql and running as the owner of the table "app" I
try to access the columns of the table like so:

SELECT * FROM app;

which returns all the columns in the table including
the one I'm interested in, which is "companyID".
If instead I use something like:

SELECT companyID FROM app;

I get the following:

ERROR: column "companyid" does not exist

even though the column DOES exist (the previous query
returned "companyID" as one of the column headers). Any
suggestions as to what I might be missing? I'm running
Postgres 7.4.1.

Regards,
Iker