Horn fly cuticular paraffin and monoolefin hydrocarbons were chemically identified and assayed for biological activity as attractants. The majority of the paraffins were odd-numbered, straight-chain molecules 21-29 carbons in length; much smaller amounts of even-numbered, straightchain molecules 22-28 carbons in length and methyl-branched compounds were also present. At least 80% of the monoolefin consisted of straight-chain molecules 23, 25, and 27 carbons in length, two of which have been identified as sex pheromones in other muscoid species. The hydrocarbon profiles among sexes and strains (laboratory and wild) were very similar except for wild females, which showed quantitative differences from the other sources. However, only females showed significant (albeit low) responses to some test materials, both synthetic and natural, and activity appeared to be centered in the monoolefins.