Where South Polar Skuas (Stercorarius maccormicki) occur in the absence of Brown Skua (S. antarcticus), their main prey consists of other birds. In contrast, where the two species occur together, fish appears to dominate the diet of South Polar Skuas, probably because Brown Skuas monopolise the terrestrial resources. Twenty-eight stomach samples of South Polar Skuas were collected throughout the breeding season at Half Moon Island, South Shetland Islands, Antarctica. Fish were the most dominant prey item (100% frequency, 98% prey mass); their importance remained constant throughout the study period. Although there is a colony of Chinstrap Penguin (Pygoscelis antarctica) nearby the South Polar Skua population and there are very few Brown Skuas, the South Polar Skuas did not eat penguins. The result suggests fish are selected preferentially over terrestrial resources, at least in certain regions, and particularly when availability of terrestrial resources is limited. Thus, competitive exclusion by Brown Skuas may not fully explain dietary choice of South Polar Skuas.