In most cooperative breeders, dominants suppress the reproduction of subordinates. However, two previous studies of Neolamprologus pulcher, a cooperatively breeding cichlid fish, have suggested that socially subordinate helper males sneak fertilizations from dominant breeding males. If such sneaking does occur, both theoretical work and empirical studies of other fish species suggest that sperm competition will select for increased reproductive investment by sneaker males, relative to more dominant males. To address these issues, we quantified gonadal investment and sperm characteristics of 41 N. pulcher male breeders and 62 male helpers from 55 groups in Lake Tanganyika. Gonadal investment followed patterns consistent with reproductive suppression, with breeders having considerably larger testes masses than helpers. Breeders also had faster and longer swimming sperm and a higher percentage of motile sperm compared to helpers. However, sperm characteristics of large helpers were similar to those of breeders, but these same helpers had lower testes masses. Thus, large helpers had sperm that were physiologically equivalent to that of breeders, but their relatively small gonads imply that they were reproductively suppressed.