Looping through an internal table

I wrote the following piece of code:

<b> LOOP AT gt_rpm_project INTO ls_rpm_project

WHERE guid = ls_cgpl_project-guid

AND category = p_categ

AND subcategory IN p_subct.

ENDLOOP.</b>

where

gt_rpm_project is an internal table and

p_subct is a select option in the selection screen.this code gives an error mentioning <b>IN cannot be used</b>. Since it is a select -option i cannot use '=' or'between'(as p_subct apart from having a range can also have individual entries.)

Please advice how to go about this.

Thanks in advance!!

Saurabh