Horn flies,Haematobia irritans(L.), significantly alter their distribution on cattle treated with pyrethroid-impregnated ear tags in comparison with untreated cattle.H. irritansprefers the largely untreated ventral and posterior regions of tagged cattle. The behavioral resistance to fenvalerate is largely a stimulus-dependent phenomenon, as indicated by the large differences in distribution patterns of a resistant horn fly population on tagged and tag-free cattle in adjacent pastures. Resistant horn flies showed a significantly greater preference for the bellies of tag-free cattle than did susceptible flies. Intensive selection by pyrethroids may also result in a horn fly population that has some degree of stimulus-independent behavioral resistance (i.e., chronic occupation of untreated areas even in the absence of a toxicant).