Effects of azadirachtin, a triterpenoid extracted from neem seed, Azadirachta indica A. Juss., were similar to those of insect growth regulators against the immature stages of the born fly, Haematobia irritans (L.), the stable fly, Stomoxys calcitrans (L.), and the house fly, Musca domestica L. When an ethanolic extract of ground seed was blended into cow manure, LC50 and LC90's for larval horn flies were 0.096 and 0.133 ppm azadirachtin, respectively. An emulsifiable concentrate (EC) had an LC50 for larval horn flies of 0.151 ppm and an LC90 of 0.268 ppm. For larval stable flies, the EC formulation had an LC50 of 7.7 ppm and an LC90 of 18.7 ppm azadirachtin in manure. Against larval house flies, the LC50 and LC90 were 10.5 and 20.2 ppm, respectively. When the EC formulation was administered orally to cattle at a rate of greater than or equal to 0.03 mg azadirachtin per kg of body weight per day or when ground neem seed was given as a daily supplement of greater than or equal to 10 mg seed per kg body weight, horn fly development in the manure was almost completely inhibited. In contrast, ground seed mixed in cattle feed at the rate of 100-400 mg seed per kg of body weight per day caused less than 50% inhibition of stable flies in the manure.