The ability of larger males to control resources and monopolise spawnings is thought to explain the benefits of protogyny in some animals. Here we describe the breeding and non-breeding social organisations of a temperate protogynous goby, Coryphopterus nicholsii, in the light of this prediction. Our results confirm that C. nicholsii has a stable social organisation, based on the year-round defence of a territory by both females and males. As predicted, larger individuals of both sexes were more likely to defend territories than smaller ones. These territories always included one or more shelter rocks, which were used as refuges by both sexes throughout the year, and also as nest sites by males in the breeding season. We then use experimental manipulation of shelter rock availability to examine the influence of intraspecific competition on territory establishment by smaller males. Following the removal of resident, non-nesting males, territories were quickly taken over. In addition, artificial reefs and nest sites were quickly colonised. Therefore, it appears that suitable shelter rocks are limited. Furthermore, intraspecific competition is strongly size-specific in C. nicholsii and determines which individuals gain access to shelter rocks. The presence of these social conditions is consistent with the prediction that the ability of larger males to monopolise more than one mate affects the occurrence of protogyny in some animals, and may clarify the occurrence of protogyny in C. nicholsii.