An avermectin, Merck MK-933 [at least 80% 22,23-dihydro-5-O-demethylavermectin A10 and not more than 20% 22,23-dihydro-5-O-demethyl-25-de(1-methylpropyl)-25-(1-methylethyl)avermectin A10], was a highly effective larvicide against horn flies, Haematobia irritans (L.), stable flies, Stomoxys calcitrans (L.), face flies, Musca autumnalis De Geer, and house flies, M. domestica L. When given to cattle daily in an oral capsule, 1 μg/kg per day killed all horn fly larvae in the manure. Daily oral dosages of 5 μg/kg killed all of the face flies, ca. 60% of the stable flies, and 90% of the house flies in the manure. Cattle dosed at 20 μg/kg per day resulted in >90% mortality of stable fly larvae in the manure. Additionally, daily subcutaneous injections of MK-933 at a rate of 5 μg/kg per day prevented horn fly development in the manure, but injections of 10 mg/kg per day did not control stable fly larvae. A single injection of 200 μg/kg, the anthehmntic dosage, controlled horn flies in the manure for up to 4 weeks posttreatment.