Three electrophoretic patterns of catalase were demonstrated in the liver extracts of the black rockfish, Sebastes inermis. All the types gave only single bands of catalase activity on the gel, but were distinguishable by mobility. Analysis of phenotype frequencies in the two samples consisting of 142 specimens indicated that those patterns are attributed to two alleles at an autosomal locus. The alleles were proved to be independent of the size of the fish, and to be maintained in the same frequency among the samples collected at different seasons. The catalase patterns were most clearly demonstrated by the negative iodine-starch reaction, and satisfactorily preserved in acetic acid. The method based on the enze-matic oxidation of aromatic amines was not successful for localizing the enzyme on the gel.