The reproductive behaviour of individually marked southern sea lions, Otaria byronia, was studied from 1981 to 1986, at Punta Norte, Peninsula Valdés, Argentina. Males defended territories along the high water mark early in the breeding season but switched to female defence as they were joined by females. Males defended females from neighbours and intruders, and forcefully held pre-oestrous females near them. Males defended territories and females against neighbours and intruders with threat displays and by fighting. Males never strayed far from females. The frequency of aggressive interactions among males was positively correlated with the number of females in oestrus. A mean of 2.8 females was associated with each male in the central breeding area at peak season, with some males being associated with 8-16 females for a few hours. Ninety four percent of the males that held a territory for five or more days copulated. Copulations were not observed away from territories in the central breeding area. Mating success was positively correlated with tenure on territory. The first males to arrive had the longest tenures and achieved the most copulations. Mean tenure on territory was 24 days (range = 15-57 days). The mean number of observed copulations per marked adult male was 3.87 ± 3.31. Variance among breeding males was low; the most successful male achieved only 7% of the copulations observed. Females gave birth a mean of 3.3 days after arrival, copulated once six days later and then went to sea to feed 7.6 days after parturition. Thereafter, females alternated periods of feeding at sea (X = 2.8 days) with periods of nursing their pups (X = 2.3 days). Males sequester pre-oestrous females. This mating strategy leads to differences in behaviour between Otaria and males of other sea lion species. These differences are evident in harem size, time away from females (as manifested by brief and infrequent fighting), the high percentage of males breeding, and the low variance in mating success among breeding males.