An infant northern fur seal (Callhorinus ursinus) died in a rookery on St. Paul Island, Pribilof Islands, Alaska. Grossly, slight enlargement of the mesenteric lymph nodes was seen. Microscopically, a lymphosarcoma composed of sheets of monomorphic lymphoid cells in sections of lymph node and tonsil was seen. Electron microscopy of formalin-fixed tissues revealed several structures that were possibly of viral origin in the cytoplasm of the neoplastic cells.