In topical applications of insecticides to adult, field-collected horn flies (Siphona irritans (L.)) in Oregon, the LD-50 and LD-90 dosages, respectively, in micrograms per fly were: DDT, 0.0058 and 0.019; methoxychlor, 0.025 and 0.072; toxaphene, 0.017 and 0.037; and malathion 0.012 and 0.04. Data are presented on the comparative susceptibility of horn flies, stable flies (Stomoxys calcitrans (L.)), and house flies (Musca domestica L.) to DDT.