In Western Canada, Aedes flavescens (Müller) is found mainly on the prairie, where it is one of the commonest of the blood-sucking pests of livestock and man. It has also been taken as far north as Churchill, Man., and Alaska (Rempel, 1950). Information on its flight range is useful in planning control programs. Field and laboratory studies on the flight range of this species, with radio-active phosphorous for tagging, were conducted at Saskatoon and Indi, Sask., in 1952. The project area is almost flat in the north and west and rolling in the south and east; the Blackstrap Valley separates the two topographical areas.