A synthetic pyrethroid (m-phenoxybenzyl cis, trans- (±)-3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropanecarboxylate) received from FMC Corporation as 2 isomer ratios designated FMC 33297 and FMC 41655 was highly effective against stable flies, Stomoxys calcitrans (L.), and horn flies, Haematobia irritans (L.), on cattle. In the preliminary test (spot test), as low as 0.0075% AI applied to a small area of a steer controlled stable flies for 8 days posttreatment. In secondary tests (large-cage test), whole-animal sprays of 0.05% AI controlled stable flies for 7–10 days and horn flies for 17–42 days. In field tests, whole-animal sprays of 0.05% AI controlled horn flies (25 or fewer flies/animal) ca. 20 days, whereas the standard, 0.5% methoxychlor, controlled horn flies less than 10 days.