The Belliidae are a small family of crabs whose relationships with other families are enigmatic and whose composition is controversial. It is by no means certain whether their genera form a monophyletic group. One member of this family is Heterozius rotundifrons, a New Zealand endemic crab that lives in the intertidal zone, sheltering under rocks. In the present study the mating behaviour of this species was examined under laboratory conditions. Mating and moulting are closely linked because H. rotundifrons females only mate immediately after moulting. Females remain attractive to males for almost 3 days. A female-derived water-borne pheromone seems to be the basis of this attraction. Prior to moulting males guard females for up to 5 days. During the female moult some males were observed to assist the female. During copulation both males and females exhibited an unusual limb quivering behaviour and afterwards males guarded the females for 10.5 h, which was extended to 37 h when other males were present. Field studies showed that females moulted during early spring over a period of 1 month. Throughout the year very few females moulted, suggesting that there may be intense competition for mates during the early spring. At all times the operational sex ratio was biased in favour of males. In terms of the evolution of reproductive characters among the Belliidae, Atelecyclidae, Cheiragonidae and Corystidae, H. rotundifrons occupies an intermediate position between the ancestral condition and that seen in Corystes cassivelaunus.