Leptospira pomona was isolated from the urine and renal tissue of a subadult male California sea lion (Zalophus californianus californianus). Serum from the sea lion agglutinated L. pomona to a titer of 1:10,000. There was marked interstitial nephritis with lymphocytic and plasmacytic infiltration. Widespread degenerative changes occurred in the tubular epithelium of the kidney, but the glomeruli appeared to be spared. Erosions and hemorrhages were present in the buccal and gastric mucosa. Hematologic abnormalities were poikilocytosis, numerous normoblasts, and leukopenia with a shift to the left. It is believed that this isolation of a leptospire is the first described from a marine mammal.