AbstractIndependent studies were performed in Canada and in Denmark to assess the survival of parasitic wasps (Hymenoptera: Pteromalidae) wintering in puparia of house fly,Musca domesticaLinnaeus (Diptera: Muscidae). Data in Canada were collected forMuscidifurax raptorGirault & Saunders,M. raptorellusKogan & Legner,M. zaraptorKogan & Legner,Nasonia vitripennis(Walker),Spalangia cameroniPerkins,Trichomalopsis sarcophagae(Gahan) andUrolepis rufipes(Ashmead) in three microsites at an outdoor cattle facility in southern Alberta. Survival was highest forN. vitripennis,T. sarcophagaeandU. rufipes, ranging from near zero toc. 7%. No survival was observed forS. cameroni. Daily mean values for ambient air temperature (DMAT) averaged about −3.5°C during exposure periods. Data for Denmark were collected forM. raptor,S. cameroniandU. rufipesin a dairy barn and in a swine barn. Survival ofM. raptorandU. rufipeswas higher than that ofS. cameroniin the dairy barn (DMAT = 8.6°C), with the three species having similar survival in the swine barn (DMAT = 15.4°C). In both studies, parasitoids in egg stages were least likely to survive. These results identify the potential forT. sarcophagaeandU. rufipesto be commercialized for use in northern climates as biocontrol agents for nuisance flies, compare directly the cold-hardiness of commercialized species (i.e. all of the above species excludingT. sarcophagaeandU. rufipes), and document the importance of microsite on winter survival.