In house flies, a leucine to phenylalanine mutation in the sodium channel (kdr) and metabolic detoxification via CYP6D1v1 have been identified as resistance mechanisms to pyrethroid insecticides. In an effort to monitor the future utility of pyrethroids, we genotyped individual flies for kdr and CYP6D1v1 from 4 collections along the eastern USA. Our results indicate a high frequency of both genes in these populations corresponding to the high levels of resistance to permethrin. The implications for resistance management are discussed.