Insects may utilize a similar complex battery of biochemical defense responses to the stimulation of insecticides as that used in respond to injury and microorganism infection. Cytochrome P450s are very important in detoxification of insecticides, hence conferring insecticide resistance. However, less is known about P450 gene regulation in response to the insecticides treatment and the factors involved in the regulation. To better understand the importance of cytochrome P450s in insecticide resistance as well as their regulation in response to insecticide challenge, a comprehensive effort has been made to characterize the P450 genes in house flies in response to insecticides treatment. A novel P450 gene, CYP6A5v2, has been cloned and sequenced. The expression of CYP6A5v2 is up-regulated by permethrin treatment. The genetic linkage analysis indicates that CYP6A5v2 is linked to chromosome 5, which is consistent with the linkage of insecticide resistance in the house fly. The possible elements involved in the regulation of CYP6A5v2 will be discussed.