Technical Abstract: The horn fly, Haematobia irritans L., is an obligate blood-feeding fly and the primary insect pest parasitizing cattle in the United States. Pesticide resistance has become a huge problem for cattle producers and although several mechanisms of resistance are possible, target site resistance is the most important resistance mechanism preventing control of this fly in the United States and possibly other countries. We developed a multiplex PCR assay to detect the known target site pyrethroid resistance-associated mutation in the horn fly and a recently reported G262A mutation in the horn fly acetylcholinesterase, the target site for organophosphates. As expected, the pyrethroid resistance target site mutation was found in fly populations from Texas, Louisiana, Washington, Georgia, Mexico and Brazil. This mutation was found to have a gender bias as it was more prevalent in females than males. The G262A acetylcholinesterase mutation was found in Texas, Louisiana, Washington, Georgia, and Mexico but not Brazil. There was no gender bias in the occurrence of this mutation, and there was no correlation between the occurrence of the kdr and the G262A mutations. Unlike the case with the pyrethroid target site mutation, this mutation did not appear to exclusively provide the level of resistance required to account for bioassay results. It is likely an additional mutation(s) occurs in the target site and/or a metabolic resistance mechanism exists in organophosphate resistant horn fly populations.