Populations ofNasonia vitripennisWalker (Hymenoptera: Pteromalidae) were collected from house flies(Musca domesticaL.) at poultry facilities in Sullivan County, New York (NY), and Zimbabwe, Africa (Af). In laboratory studies, the two populations parasitized approximately the same number of hosts, but more NY strain progeny emerged per host pupa than did Af strain progeny. Oviposition rates were similar between the two strains, but a larger number of Af individuals died during development within the host pupae. A greater number of diapause larvae was produced by the NY strain. The results of this study emphasize the importance of understanding intraspecific differences in insects used as biological control agents.