Organophosphate (OP) insecticides (parathion/diazinon) resistance in housefly (Musca domestica L.) is associated with the change in carboxylesterase activity. The product of alphaE7 gene, which is a member of alpha-esterase gene cluster, is probably playing a role in detoxyfication of the xenobiotic esters. In parathion/diazinon resistant M. domestica species Gly(137) to Asp substitution was found in the active center of the product of alphaE7 gene. In malathion (an OP) resistant M. domestica strains Trp(251) to Ser substitution was identified in the active center of the MdalphaE7. In our research, to understand the allelic diversity of the MdalphaE7, the gene was partially sequenced from four different housefly strains from different localities (Guatemala, Manhattan (USA), Colombia (USA), and Thailand). It was found out that; in Thailand strain one allele has Cys residue at the position of 251, the other allele contains a Trp for the same site. In Colombia strain, one allele has Asp(137), the other allele contains a Gly residue at this point. The Manhattan and Guatemala strains have Asp(137) and Trp(251) residues on their both alleles at these two different positions.