Manure from cattle treated with an intramuscular injection of an avermectin, MK-933 [at least 80% 22, 23-dihydro-5-O-demethylavermectin A1a and not more than 20% 22,23-dihydro-5-O- demethyl-25-de(l-methylpropyl)-25-(1- methylethyl)avermectin A1a], at a rate of 200 μg/ka prevented emergence of adult horn flies, Haematobia irritans (L.), from all collections through 28 days posttreatment. Some adult horn flies emerged from manure collected at 35 days posttreatment and then aged 2 weeks before infestation. Emergence of several endemic insects was severely reduced in the manure of treated animals; however, this treatment did not influence emergence of all the insects. Manure from treated animals appeared to physically disintegrate at the same rate as that from untreated animals