House fly control with treatments of malathion baits during 1956 at 1 dairy in the Savannah, Georgia, area provided excellent abatement until Aug., at which time fly populations began to increase rapidly. Field and laboratory tests showed that these flies were not attracted to the 4 types of malathion bait formulations and in essence appeared to be repelled by the bait. Flies at other dairies in the general area, and in Florida, were readily attracted to all the malathion formulations tested. Laboratory tests using topical applications also showed this strain of flies to be resistant physiologically.