The influence of various environmental factors on flight activity ofWinthemia fumiferanaeTothill (Diptera: Tachinidae) was studied in southern Québec from 1985 to 1987. Adults of both sexes were strictly diurnal and much more active in the presence of sunlight. Flight activity was positively related to maximum air temperature above 15°C and, to a much lesser degree, negatively related to precipitation. Males and females showed different spatial distribution patterns. Males were more numerous in the lower canopy of the forest, whereas females were more abundant in the higher canopy. Males also were found more often in deciduous vegetation than on conifers, the inverse being observed for females. We conclude that sylvicultural practices that might enhance habitat diversity and natural lighting should favor the activity ofW. fumiferanaeas a parasitoid of the spruce budworm,Choristoneura fumiferanaClemens (Lepidoptera: Tortricidae).