Polyembryony is a unique mode of development in which multiple genetically identical embryos develop from a single egg. In some polyembryonic species a proportion of the embryos develop into soldier larvae, which attack competitors in the host. We studied the development of the polyembryonic wasp Copidosoma koehleri in its host Phthorimaea opercullela. We dissected hosts parasitized by either virgin or mated female wasps at 2day intervals from hatching to the final instars. We documented host mass and head width, the number and size of developing wasps and the presence of a soldier larva. Additionally, we kept a sample of parasitized hosts until emergence of wasps and measured the head width of emerging adults. We characterized wasp development in relation to host development. One half of the broods produced by mated wasps contained one soldier larva throughout development. This suggests that in C. koehleri each female brood produces a single soldier larva, and that the soldier probably survives and grows gradually during host development. Additionally, we found that female broods were larger than male broods during development and also upon emergence. Accordingly, body size was larger for males during development as well as upon emergence. These findings may extend the existing knowledge on polyembryonic development in general, and serve as a baseline for further experiments.