Cytochrome P450 Ipr is a developmentally regulated P450 responsible for monooxygenase-mediated pyrethroid resistance in the LPR strain of housefly. CYP6D1, the gene coding for P450 Ipr, has recently been sequenced. We investigated the molecular basis for CYP6D1-mediated pyrethroid resistance by comparison of mRNA levels, gene sequences, and gene copy number between LPR and pyrethroid susceptible strains of housefly. CYP6D1 mRNA levels were elevated in the LPR strain to a similar level as P450 Ipr protein, suggesting that over-expression of this important P450 in the resistant strain is probably due to increased transcription. Southern blots of susceptible and LPR strain DNA suggest that gene amplification is probably not involved in the increased expression of CYP6D1 protein. Five alleles of CYP6D1 were discovered and their possible role in resistance is discussed.