Brood size has important implications for the fitness of both parents and offspring. In polyembryonic parasitoid wasps, each egg develops into many genetically identical embryos through clonal division inside the host. Thus, offspring may have the potential to affect brood size by adjusting the degree of embryonic division. In some species, a proportion of embryos develop into soldier larvae, which attack competitors inside the host. This may be another mechanism for offspring to affect final brood size. We investigated the effect of relatedness between competing clones on brood size in the polyembryonic wasp Copidosoma koehleri. We predicted that final brood size would be affected by the number and relatedness between competing clones inside the host. Additionally, we predicted that due to a competitive asymmetry between male and female clones (apparently only female clones produce a soldier larva), this effect would depend on the sex composition of wasps inside the host. We allowed 2 wasp eggs (laid either by 1 female or by different females) to develop in a host and counted the emerging adults. Relatedness between male clones did not affect brood size. However, female-containing broods of related clones were larger than broods of nonrelated clones, suggesting higher aggression of the soldier toward less related individuals. Dissections of hosts parasitized by 2 clones indicate that normally only 1 soldier survives and that it often eliminates unrelated clones. Thus, offspring control over brood size in response to relatedness is probably mediated by soldier aggression and not by clonal division.