Pyrethroids are a widely used class of insecticides utilized for control of a vast number of arthropod pests. A significant limitation to the long-term use of pyrethroids is the development of resistance. In housefly, the two most important mechanisms of resistance to pyrethroids in the USA are kdr (insensitivity of the voltage sensitive sodium channel (VSSC) to pyrethroids) and CYP6D1-mediated detoxification. In house fly, kdr is due to a single amino acid (L1014F) change in the VSSC. A second mutation (M918T), called super-kdr has been identified in house flies from Europe and Japan, but not the USA. Another mutation has been found in house flies (L1014H), which also confers pyrethroid resistance. We evaluated the frequency of VSSC1 and CYP6D1 alleles across two states and over two years. Although the levels of resistance do not change substantially, there are remarkable changes in allele frequencies. The importance of fitness costs of these resistance alleles will be discussed.