Abstract: Synergism of malathion with cypermethrin, phosalone, phorate and fenitrothion was tested against Musca domestica L. Malathion and other insecticides were mixed in the ratios of 1:9, 3:7, 5:5, 7:3 and 9:1 and applied topically on adult houseflies and the combined LD50 and co-toxicity of co-efficient were calculated after 24 h of treatment. The unsynergised malathion, cypermethrin, phosalone, phorate and fenitrothion gave topical acute LD50 of 606.81, 31.83, 203.95, 153.98 and 209.50 ng/fly respectively. The combined action of malathion with cypermethrin offered LD50 values of 6.94, 12.71, 22.44, 49.50 and 187.17 ng/fly; with phosalone 27.76, 95.31, 201.90, 411.56 and 1779.42 ng/fly; with phorate 2.40, 6.26, 67.99, 88.18 and 264.65 ng/fly; with fenitrothion 37.59, 111.66, 158.94, 320.35 and 433.79 ng/fly for 1:9, 3:7, 5:5, 7:3 and 9:1 ratios respectively. Among the tested pesticides, malathion found least toxic and in case of phorate, the degree of potentiation increased with the increase of its proportion in the mixture. The investigation revealed that the malathion could be potentiated by mixing other pesticides