House fly,Musca domesticaL., pupae parasitized by three species of pteromalid wasps,Spalangia cameroniPerkins,Urolepis rufipes(Ashmead), andMuscidifurax zaraptorKogan&Legner, were released into house fly and stable fly,Stomoxys calcitrans(L.), breeding habitats on three dairies during the spring season. In conjunction with these releases, freeze-killed house fly pupae were placed in the same areas to serve as hosts for the emerging parasites.S. cameroniandU. rufipesdid not use the freeze-killed pupae as hosts, butM. zaraptorreadily oviposited on and produced progeny from the introduced hosts. Freeze-killed pupae remained suitable as hosts forM. zaraptorfor up to 4 weeks at the cooler spring temperatures. Large numbers of second-generation progeny after 8 weeks suggested that the use of early spring releases ofM. zaraptorand freeze-killed hosts may effectively build up early season populations of this pteromalid species for the control of house flies and stable flies present on midwestern livestock confinements.