A California sea lion (Zalophus californianus) exhibiting recurrent ulcerative skin lesions was biopsied for viral isolation by explantation. Fibroblastic outgrowth of explanted skin developed large areas of multinucleated syncytia. Electron microscopy demonstrated virus particles characteristic of spumaviruses in the family Retroviridae. Similar skin lesions developed on a harbor seal (Phoca vitulina) about one month after introduction into the sea lion exhibit. Scattered areas of multinucleated syncytia developed in the explanted skin fibroblasts from this animal, though virus was not recovered. No syncytia developed in fibroblasts from a harbor seal which had no history of skin lesions and no contact with the sea lion exhibit. Skin lesions reoccurred in the otherwise asymptomatic sea lion 4 months after the initial biopsy. The animal was rebiopsied and died suddenly 4 days later after 24 hours of anorexia and pyrexia. At post mortem, death was attributed to a Pasteurella multocida pericarditis and septicemia. Lung, kidney, spleen, and lymph nodes were explanted in an effort to determine the extent of the spumavirus infection. Cellular outgrowths of the lung occasionally developed syncytia but more frequently were lysed. Elimination of the cell culture by EM showed virus particles characteristic of a herpesvirus. Spumaviruses are not known to cause clinical disease. They do cause a persistent infection of the reticulo-endothelial system possibly altering the host's immune response. Herpesviruses cause a wide variety of clinical diseases and latent infections. Neither spumaviruses nor herpesviruses have been previously described in pinnipeds. The role of these viruses in pathogenesis of disease in pinnipeds remains speculative, but either might predispose an animal to a fatal secondary infection.