The adult stage of the horn fly, Haematobia irritans (L.) is a permanent haematophagus parasite of cattle throughout the United States. Populations may exceed 1,000 flies per animal in some areas. Nationally, the loss due to this ectoparasite is estimated to be 730 million dollars per year (1). Heavy populations decrease weight gains 0.10 lbs per day and decrease feed conversion efficiency about 9% (2). Historically, the horn fly was controlled by various types of insecticide treatments applied as sprays or dips, or by self-treatment devices such as cable rubbers or dust bags. Insecticide-impregnated cattle ear tags rapidly became the treatment of choice in the 1980's because they provided efficient, economical and season-long control with one treatment. Unfortunately, ear tag treatment provided an ideal mechanism for development of insecticide resistance. Within a few years following the introduction of pyrethroid-impregnated ear tags, horn flies began to show widespread resistance to this entire class of insecticides (3). Hom fly control for the future should rely on strategies that minimize exposure of horn flies to a single class of insecticide, and particularly to the pyrethroids. Studies reported herein show the efficacy of topically applied ivermectin against the horn fly on pastured cattle. In addition, studies evaluating the persistence of topical ivermectin treatments on animals exposed to controlled environmental conditions are presented.