Three cases of skeletal muscle neoplasms occurring in two small fish species used in carcinogen tests are reported. The cases illustrated a wide range of histologic patterns including a well-differentiated juvenile type, a well-differentiated pleomorphic type, and a poorly differentiated pleomorphic type. A rhabdomyosarcoma with juvenile type features developed solvent dimethylformamide. Single cases of pleomorphic type rhabdomyosarcoma also occurred in the medaka and in the guppy, Poecilia reticulata, exposed to the carcinogen methylazoxymethanol acetate. These cases indicate the potential for skeletal muscle cells in medaka and guppy to become neoplastic, but the low frequency of the tumors does not necessarily establish a chemical etiology.