Recent studies of fur seals and sea lions (Otariids) which are cold water adapted, indicate that social behaviour in hotter climes is constrained by the form and availability of cool substrate provided by the rookery environment. This first study of the behaviour of the Juan Fernández fur seal provides new evidence of this relationship and the extent to which thermoregultory requirements can affect social behaviour in pinnipeds. Females of this species on Alejandro Selkirk Island made daily movements from inland pupping and rest sites to the shoreline and into the water in response to rapid increases in solar radiation to levels exceeding 1.3 cal/cm2/min. Thirty percent of these females floated and groomed offshore in the afternoon in areas protected from the surf by offshore islets and rocky reefs. Males held territories on land either along the shoreline (39%) or in land-locked areas (45%) as is typical of fur seals, or held completely aquatic territories (16%) that encompassed the site where females floated. The average aquatic male, present on territory primarily during the afternoon hours when females floated offshore, achieved as many copulations as did the average land-locked or shoreline male. Land-locked males often abandoned their territories for short periods (45 minutes on average) in response to increasing solar radiation in the afternoon and at a time when female numbers on land were low. The existence of aquatic territoriality as a successful mating strategy has not yet been documented for any other otariid. The occurrence of this behavioural strategy is likely a product of the interaction of thermoregulatory constraints and topographical features which together promote predictable aggregations of females offshore. This study further substantiates that constraints on female aggregation appear to dictate the range and mode of male mating strategies in otariids.