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. We investigated the effect of relatedness between competing clones on brood size in the polyembryonic wasp Copidosoma koehleri.