South American fur seals Arctocephalus australis and South American sea lions Otaria fiavescens breed sympatrically on several islands of the coast of Uruguay. O. fiavescens males will sometimes attack A. australis pups and shake or toss them for several minuh:s or even hours. These attacks may result in the death of the pup. I recorded 31 pup attacks at Rasa island, Uruguay, during 1996 breeding season. Most abductors were subadults, which frequently engaged in aggressive encounters with each other during the abduction. After these encounters, the largest males always retained the pups. Forty percent of the pups died during the attacks, which normally ended when the abductor abandoned the pup. Abductors did not mount or eat the pups. Attacks by Otaria males did not appear to have the function of killing Arctocephalus pups. Instead, inter-specific infanticide appeared to be a by-product of intra-sexual competition or sexual frustration of subadult males. The mothers attempted to recover the pup in 31% of the attacks. However, only one mother was able to get her pup away from the abductor. This active role of the mothers was not observed in other colonies where Otaria subadults also attack Arctocephalus pups, e.g. in Peru. This difference is probably associated with the fact that in Uruguay both species use the same islands for reproduction, while in Peru they reproduce allopatrically.