In 1995, Nasonia vitripennis (Walker) (Hymenoptera: Pteromalidae), a commercially available pupal parasitoid of the house fly, Musca domestica L., and stable fly, Stomoxys calcitrans (L.) (Diptera: Muscidae), was purchased to examine the status of wasps being sold to Manitoba producers. Percentage of pupae parasitized, numbers of parasitoids per pupa, total parasitoids, and parasitoid sex ratio were determined for each shipment of parasitoids received. To determine the extent to which these wasps could successfully parasitize house flies and stable flies, parasitoids were released weekly in four Manitoba dairy barns and levels of parasitism estimated. In 10 622 freeze-killed sentinel house fly pupae, 2.2% were parasitized throughout the season by N. vitripennis, and 5.8% were parasitized by eight other species of parasitoids. Of 11 897 naturally occurring house fly and stable fly pupae, 0.6% were parasitized by N. vitripennis, and 3.4% by eight other species of parasitoids. In four barns where there were no releases of N. vitripennis, 1.1% of 11 779 sentinel pupae were parasitized by four species of parasitoids and 3.8% of 8384 naturally occurring house fly and stable fly pupae were parasitized by nine species. The release of an estimated 3 648 093 N. vitripennis did not result in substantial parasitism in either sentinel pupae or naturally occurring pupae. In 1996, live sentinel house fly pupae (n = 50 842) and house fly and stable fly pupae occurring naturally (n = 4691) were collected in two of the nonrelease barns from the 1995 study to examine the activity of endemic parasitoids. Of the sentinel and naturally occurring pupae sampled, 4.0% and 9.4% were parasitized, respectively. Phygadeuon fumator Gravenhörst (Hymenoptera: Ichneumonidae) was the most abundant parasitoid, accounting for 97.4% and 79.9% of parasitoids collected from sentinel pupae and naturally occurring pupae, respectively. Other parasitoids included Urolepis rufipes (Ashmead), Muscidifurax raptor Girault and Sanders, Muscidifurax zaraptor Kogan and Legner, Spalangia subpunctata Först, Spalangia cameroni Perkins, Spalangia nigra Latreille, and a species of Trichomalopsis Crawford (Hymenoptera: Pteromalidae).