Preliminary bioassays with the horn fly,Haematobia irritans (L.), suggested that a mating stimulant pheromone was involved in horn fly courtship behavior. Virgin males readily courted live or dead females but rarely courted live males, dead males, or females thoroughly washed with hexane. The bioassay used was the response of virgin males to treated male horn flies. Results indicated that female cuticular hydrocarbons induced male courtship behavior. Specifically, the female paraffin and monoolefin fractions were biologically active when bioassayed alone or in combination. Three synthetic monoolefins previously shown to be the major components in the female monoolefin fraction were active in bioassays. The compoundsZ-5-tricosene,Z-9-pentacosene, andZ-9-heptacosene were each active, but greater male courtship behavior was observed when these three compounds were bioassayed in combination.