Some species of marine Isopoda collected in southern Brazil mostly in shallow-water and amongst seaweeds are studied. Five species are discussed and illustrated. A diagnosis is given for each species. Idotea metallica and Synidotea marplatensis are new oc currences for the area, and both have had their geographical range extended to off Rio de Janeiro. The presence of Idotea balthica and Erichsonella filiformis in southern Brazil was definitely asserted. Illustrations of both sexes of Rocinela signata and details about its occurrence in the region surveyed were made.