Organophosphate insecticide (parathion/diazinon) resistance in housefly ( Musca domestica L.) is associated with the change in carboxylesterase activity. The product of MdalphaE7 gene is probably playing a role in detoxification of xenobiotic esters. In our research, we have isolated, cloned and sequenced the MdalphaE7 gene from five different Turkish housefly strains. High doses of malathion ( 600 mug/fly) were applied in a laboratory environment for one year to Ceyhan1, Ceyhan2, Adana, and Ankara strains while no insecticide treatment was performed in the laboratory to Kirazli strain. Trp(251) --> Ser substitution was found in the product of MdalphaE7 gene in all malathion-resistant and Kirazli stocks. In addition, we checked the malathion carboxylesterase (MCE), percent remaining activities in acetylcholinesterase (AChE), glutathion-S-transferase (GST), and general esterase activities in all five strains used in this study. In comparing with universal standard sensitive control WHO, a high level of MCE and GST activities were observed while lower level of general esterase activities was detected in the tested strains. In addition, a higher percent remaining activities in AChE than WHO susceptible strain were observed in all malathion-resistant strains.