AbstractThe toxicity of 1,3‐di(methoxycarbonyl)prop‐l ‐en‐2‐yl dimethyl phosphate (‘Bomyl’) to female houseflies (Musca domestica L.) was compared with that of the carbamate insecticide methomyl. Insecticide‐susceptible (Cooper) and multi‐insecticide‐resistant (G and GIV) strains of flies were tested by exposure to granular bait and by topical application of technical formulations. Females from G and GIV strains were shown to be resistant to bomyl and methomyl, and resistance levels were related to the insecticide and test method used. Resistance ratios for bomyl and methomyl were low for the GIV strain when the toxicants were applied topically, being x6.1 and x 9.3 respectively at the LD95 level, but when sugar granules were coated with the insecticides and fed to flies, the resistance ratios were higher at x 43.0 and x 15.6. Flies from the resistant G strain were also shown to be resistant to the granular bait formulations of bomyl and methomyl when the granules were scattered or suspended from treated bait targets. A small proportion of flies from the resistant G and susceptible strains recovered from knockdown after feeding on methomyl granules, but most of these had lost their ability to fly. There was no significant recovery of flies in any of the bomyl tests. The feeding responses of individual female flies to scattered bomyl granules was also studied. Flies from the resistant G strain took significantly longer than susceptible flies to initiate a feeding response (P<0.001). Once feeding had commenced there was no difference between the strain