Fur seal populations have declined since the late 1950's. In order to assess if pup mortality was a significant contributor to the decline, causes of pup mortality were exlamined. Approximately 149Q Callorhinus ursinus pups ranging in age from fetuses to 12 weeks were necropsied on the Pribilof Islands (St. Paul and St. George), Alaska during July and August, 1986-1993. Pups were collected each day from rookeries and necropsied. Primary causes of death included emaciation (46%), stillborn (7%), premature births (0.6%), trauma-blunt (16%), trauma-sharp (5%), infections (5%), fetal anomalies (1%), miscellaneous/undetermined/no gross lesions (14%), and White Muscle Syndrome (wms) (3%). The WMS was characterized by multifocal necrotizing myositis and myocarditis and occurred during 1990 and 1991. The cause of the WMS was not determined but the epizoology and lesions were suggestive of an environmental toxin. Two neoplasms (ganglioneuroblastoma and an unidentified skin tumor) were found. Hookworm disease was important during the 1950's and 60's when the population was at a maximum. Now, however, hookworms are only occasionally found and hookworm disease has not been found in this study.