A male sub-adult California sea lion (Zalophus californianus) was found beached on the north coast of California in June 1980. The animal presented slightly depressed with some respiratory congestion, unable to ambulate with its hind-flippers and exhibiting old deep necrotic lesions on both hind-flippers. The animal weakened and expired four days post-admission. Gross necropsy examination revealed bilateral, caseous masses anterior to the ureteral orifices of the bladder wall. Metastatic tumor lesions were located elsewhere in the body. Histopathologic diagnosis was metastatic transition cell carcinoma. Details of this case study will be discussed.