The polygynous New Zealand fur seal, Arctocephalus forsteri, breeds in densely-packed colonies with males defending exclusive territories. The distribution of individuals 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. Shaded areas and pools of water were used by both sexes 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. Using the number of females on a territory as a measure of male mating success, it was found that female numbers were most strongly positively correlated with the area of shaded substrate on a territory. Total territory area was also positively correlated with number of females. It is argued that thermoregulatory constraints are a major factor affecting female site choice, and that this in turn affects male mating success.