Infant abuse by males has been observed in many pinniped species, but its adaptive significance and defense mechanism remain uncertain. We studied harassment and abduction of pups by nonterritorial male northern fur seals (Callorhinus ursinus) on St. Paul Island, Alaska, from 1993 to 1998. Juvenile, subadult, and adult males entered the breeding area and sniffed, bit, grabbed, or mounted pups. They also abducted pups to other places on land or to sea. Three pup mortalities caused by drowning, skull damage, or separation from the mother leading to emaciation were observed during the study period. Nonterritorial males did not exhibit a preference for the sexes of pups they attacked, and never consumed dead pups. Intrusions of juvenile and subadult males into breeding areas increased in the late breeding season, when harassment and abduction of pups occurred frequently. Frequency of intrusion and harassment of pups by adult nonterritorial males was lower than that by juvenile and subadult males, and did not increase in the late breeding season. On average, each pup was harassed or abducted 3.8 times in a breeding season. Pups changed their behavior to avoid nonterritorial males as pups moved out of the central breeding area in the late breeding season. Territorial males protected pups indirectly through territory defense, but their vigilance against juvenile and subadult males diminished in the late breeding season. Adult females protected their pups against juvenile and subadult males only while they were attending pups within the breeding territories. However, formation of breeding aggregation in rocky habitats may contribute to reducing the risk of harassment of pups by nonterritorial males because it provides pups with protection by territorial males as well as shelters created by irregular terrain.