Prevalence of leptospirosis was determined in California sea lions (Zalophus californianus) stranded live along the central California (USA) coast between January 1981 and December 1994. Clinical signs of renal disease were seen in 764 (33%) of 2338 animals examined; 545 (71%) of these 764 animals died, with similar gross lesions of nephritis. In silver impregnation stains of sections of formalin-fixed kidney, numerous loosely coiled spiral organisms were observed. Leptospira pomona kenniwicki was cultured from four kidney samples in 1991. Epizootics of leptospirosis occurred in 1984, 1988, 1991, and 1994, and were more common in the autumn, typically affecting juvenile males. In 1991 and 1994, 47 animals sampled had antibody titers to L. pomona greater than 1:3200. In 1992, 20 animals sampled were seronegative, and in 1993 three of 20 animals sampled had low titers to L. pomona.