We studied biochemically and cytologically the contribution of lysosomal hyrdolytic pathway to glycogen degradation in the striated muscle of diploid (control) masu salmon Oncorhynchus masou. The data were compared with those of triploid masu salmon or mouse skeletal muscle. Acid α-glucosidase activity, when measured using maltose or glycogen as substrate, was about 10 times higher in diploid masu salmon than in mice, but it was 50% lower inversely when 4-methy-lumbelliferyl α-D-glucoside (4MU_αG) as substrate. Total phosphorylase activity in masu salmon muscle was about 7-fold lower than that in mouse muscle. The result suggests that lysosomal hydrolytic pathway of glycogenolysis is relatively active in the fish muscle. We observed glycogeno-somes (glycogen-containing autophagic vacuoles) in the masu salmon muscle but not in mouse muscle, mimicking cytology of human glycogen storage disease type II (GSD II) lacking lysosomal acid α-glucosidase. However, the mechanism of the glycogenosome formation in masu salmon muscle cell can not be explained analogically as in GSD II, since acid α-glucosidase activity was not deficient. There was no clear differences in ultrastructure of the muscle cell or biochemical measurements between diploid and triploid masu salmon.