Sixty-six (18%) cases of widely metastatic carcinoma of probable transitional cell origin were identified in 370 California sea lions (Zalophus californianus) stranded alive along the central California (USA) coast, between January 1979 and December 1994. Live animals were usually emaciated and anorectic, with perineal edema and occasionally hind-flipper paralysis or paresis. Large yellow caseous masses were observed in the sub-lumbar lymph nodes, often extending around the ureters resulting in hydroureter. Histologically, metastases were usually widespread, and the primary neoplastic focus undetectable. This is the highest reported prevalence among necropsied animals of neoplasia in a pinniped population to date.