A highly active esterase associated with resistance to temephos, chlorpyrifos, parathion, methyl parathion, malathion, and fenthion in the southern house mosquito from California is coded by a single gene with 2 alleles: Est-BA, coding a strongly staining esterase; and Est-BO, coding a weakly staining esterase. Est-BA was found to be dominant over Est-BO. Est-B gene is localized on the same chromosome as the gene coding resistance and there is a strong possibility that these 2 genes are identical since no recombinants were observed.