Sea lions aborting on San Miguel Island, California, and fur seals on St. Paul Island, Alaska, were studied for the presence of infectious disease agents. Leptospira were isolated from both groups and may have been one cause of reproductive failure in both species. From a total of seven virus isolations made, one isolate from fur seals and two isolates from sea lions appear antigenically related by serum neutralization tests. In their host range, morphology, and physicochemical properties, the virus isolates are indistinguishable from Vesicular Exanthema of Swine Virus. Six mycoplasma isolations have been made but have not been fully characterized. A fungus, Scopulariopsis sp., isolated from three different sea lions, is the same genus that was repeatedly isolated from Navy divers during prolonged submergence studies.