During an ongoing physiological ecology study on pups and adult female northern elephant seals (Mirounga angustirostris, Gill, 1866) on the mainland rookery at Año Nuevo State Reserve (California), an opportunity was afforded to collect fresh dead pups for parasitology and necropsy. The investigation was undertaken to delineate the causes of death of northern elephant seals recovered from Año Nuevo State Reserve. Prior to this study, there was no evidence of increased mortality or health problems on this rookery. Necropsies, histology, and ancillary diagnostic studies were conducted on 21 fresh dead preweaned pups. Ages ranged from 1 stillbirth to pups approximately 2 weeks of age. Gross lesions included varying degrees of bruising, hemorrhage, lacerations, and fractures attributed to blunt force trauma to the head, chest, and/or abdomen in 16 pups; starvation in 6 pups; bite wounds in 2 pups; generalized icterus in 2 pups; presumptive drowning in 2 pups; and 1 stillbirth. Most pups had multiple gross lesions. Following light microscopic examination, pups could be assigned into 4 general diagnostic categories: 1) trauma, 2) nutritional status, 3) infectious conditions, and 4) congenital anomalies. This investigation of preweaned pup mortality of northern elephant seals in California further refines diagnostic categories for perinatal pup mortality.