AbstractFemale Nasonia vitripennis (Walk.) discover large housefly (Musca domestica L.) puparia more easily than small ones, and select for drilling a larger percentage of the large ones that they discover. If exposed only to small hosts instead of large ones, the parasites attack more of them in a given time, laying fewer eggs on each. This probably results because changes in parasitized hosts, leading to rejection by the females, occur sooner in small than in large fly pupae. Survival of immature N. vitripennis and of another pteromalid, Muscidifurax raptor G. & S., is greater on large hosts than on small ones. Sex ratio of both parasite species is unaffected by host size.