Large male helpers in the cooperatively breeding cichlid Neolamprologus pulcher gain reproductive success by parasitizing the reproductive effort of male territory owners. Under controlled, experimental conditions we examined the genetic relatedness between the members of brood pairs (n = 14), their male helpers (n = 8), and offspring (n = 292) in seven families. We used multilocus DNA fingerprinting to check for potential reproductive parasitism by male helpers and to assess their fertilization success. Of offspring produced in these families, 10.3% were sired by helpers. In parasitized broods, helper fertilization success varied between 12.5% and 35.8%. Male helpers parasitized parental reproduction when their body size exceeded 4.5 cm standard length (SL), even though sexual maturity may be reached much earlier (3.5 cm SL). Two of three parasitic helpers were punished by severe aggressive attacks when parasitizing the reproduction of breeders, which led to their expulsion from the territory. This study demonstrates a potential fitness benefit to broodcare helpers that is often neglected. It also points to the delicate balance that may exist between cooperative and competitive behavior in cooperatively breeding species.