Little is known about sperm allocation patterns in crustaceans, especially in anomuran crabs. We investigated whether male stone crabs, Hapalogaster dentata, change the number of ejaculated sperm depending on risk of sperm competition, and whether the pattern of sperm allocation to females of different body size varies with male body sizes in laboratory experiments where the male:female sex ratio (SR) and body size of both sexes in mating pairs were controlled. The number of ejaculated sperm differed significantly between matings under SR of 1:1 and 2:1, and males showed an increase in number of ejaculated sperm in the presence of a potential rival male. These results suggest that sperm competition may occur in the stone crab, and superiority in number of ejaculated sperm relative to rival males would be important for increasing male reproductive success in this species. Larger males showed a significant increase in the number of ejaculated sperm with increasing female size, while smaller males did not. The size of the ejaculate passed to larger females by smaller males was significantly reduced, resulting in low fertilization rates of larger females. The ability of males to provide sufficient sperm for fertilization may be one factor resulting in preference of female stone crabs for larger males.