Laboratory and field strains ofMuscidifurax raptorGirault and Sanders and a laboratory strain ofSpalangia endiusWalker were tested in environmental chambers. The field culture ofM. raptorwas obtained by exposing pupal house flies,Musca domesticaL., to a natural population of parasites.Parasitism of house fly pupae was less successful at either of the lower temperature conditions (spring/fall or winter). At summer conditions, the field culture ofM. raptoremerged over a slightly longer period than did the laboratory culture, though emergence began at about the same time. At spring/fall conditions, the field culture took somewhat less time to develop than the laboratory culture; at winter conditions, it developed much faster than the laboratory culture.Spalangia endiustook somewhat longer for development at all temperatures.