Early in the summer of 1949 reports were received that in various parts of Ontario and Quebec, particularly about dairy barns and stables, DDT residual sprays were not controlling the house fly, Musca domestica L. A preliminary investigation of these areas during that summer confirmed chat, in most instances, DDT applied as a residual spray was not giving satisfactory control. The DDT residual applications had been used at these dairy farms since 1946 and, in general, the insecticide had been properly applied. NevertheIess, a decline in control was noticed during 1948, even though the insecticide was applied more frequently than in previous years. Accordingly, studies were conducted on collections of these flies to ascertain whether shains of flies resistant to DDT had developed in Canada and, if so, to what degree.