The polygynous New Zealand fur seal, Arctocephalus forsteri, breeds in densely-packed colonies with males defending exclusive territories. The distribution of animals within a colony of these seals was monitored over three consecutive breeding seasons and behavioural thermoregulation was found to be an important factor influencing the site selection of both males and females. Three substrates were available to seals: rocks in the sun, rocks in the shade, and pools of standing water. The shaded areas and pools were used for cooling and use of these cooling substrates increased as rock surface temperature increased, resulting in a clumped distribution on hot days. Substrate preferences were tested experimentally by manipulating the availability of cooling substrates during two seasons. The number of females using an area increased significantly after shade or pools of water were added to that area. For males, pool additions resulted in increased use of those areas, while adding shade had no effect. Cooling substrates had a patchy distribution and could, therefore, be monopolised by territorial males. The physical characteristics of territories were compared with the numbers of females per territory to determine if harem size was related to cooling substrate availability. Harem size was positively correlated with both shaded substrate area and total territory size, thus demonstrating that female thermoregulatory constraints are a major factor affecting male mating success. Substrate manipulation experiments also revealed that harem size varied in relation to changes in cooling substrate availability, both within the territory being measured and on sites adjacent to it. Harem size increased on two territories after shade was added to them, and also on two territories near sites from which cooling substrates were removed. Another two territories showed a decrease in harem size after cooling substrates were added to neighbouring areas. These results suggest that the quality of a territory (i.e. number of females on it) is directly related to its cooling potential. The role of female aggression in the seals' social system was also investigated. Females defended small territories within male territories using displays and vocalisations not unlike those used by males during territorial disputes. Agonistic interactions occurred most often on shaded rocks, suggesting that this is a limited resource being contested. It is proposed that this competition for preferred microhabitats will limit harem size and, therefore, the mating success of territorial males. Resident females won significantly more conflicts than intruders, regardless of the type of substrate on which the interaction took place.