1. Up to 50% of weaned northern elephant seal pups, Mirounga angustirostris, present on the Año Nuevo mainland rookery at the end of the breeding season showed evidence (tooth marks and injuries on the neck) of having been mounted by males. 2. Male-inflicted injuries on weanlings ranged from superficial tooth marks to deep gashes and punctures that bled and exposed the blubber. Most marks were superficial. 3. Approximately 0.5 % of all weaned pups died on the rookery from 1969-1990; about 35 % of the dead weanlings showed physical evidence that they were killed by males and males were suspected of having killed most of the rest. 4. Males mounting weanlings displayed many behaviors that are characteristic of male sexual behavior toward adult females and most weanlings responded like non-estrous females. Weanling mounts lasted an average of 2.8 min (N = 81) and were usually terminated when the male did not pursue an escaping weanling. There was no bias in the sex or the size of mounted weanlings. 5. Males also mounted conspecific juveniles (one and two years old), although these interactions were relatively infrequent because most juveniles were at sea during the breeding season. The behavior of males and juveniles during mounting attempts was qualitatively similar to weanling mounts; rarely, yearlings were killed by males. 6. Most mounters (91 %) were subadult males and were observed to mount weanlings only once in a breeding season. Occasionally, a male persisted in mounting weanlings repeatedly, within a season and across years. Weanling mounters did not differ from other males in their proximity to harems or in their copulatory success. 7. The proximate factors leading to weanling mounts appear to be male sexual inexperience, high libido, limited access to adult females, and stimulus generalization. Functionally, male sexual behavior toward weanlings appears to be a low-cost by-product of high male sexual motivation that may enhance a male's reproductive success. Weanlings exhibit behaviors, such as grouping into pods, seeking microhabitat refuges, and resisting mounts, that may reduce the chance of being mounted by a male or of being injured during a mounting attempt.