Malathion resistance in house fly (Musca domestica L.) strains from Phoenix, Arizona, and Savannah, Georgia, was confirmed in laboratory tests during the fall of 1956 Physiological resistance was shown using exposures to malathion-sugar residual applications and topical applications of malathion-acetone solutions. Using colony cages with 5, 25, and 45% of the available resting surface treated with malathion-sugar residual deposits, strains with higher levels of resistance were selected from the resistant filed strain. The physiologically resistant strains also showed a correlated behavioristic resistance in their avoidance of malathion bait but not of Dipterex baits. Comparison of resistance levels of 1-, 3-, and 5-day-old adult males and females showed a marked inverse relationship of age to resistance.