Esterase amplification is the major organophosphorus (OP) insecticide resistance mechanism in Culex mosquitoes. The amplified Estα21\Estβ21 esterases are found in > 90% of resistant populations worldwide, whereas amplified DNAs (amplicons) containing Estβ1s are much rarer. Individuals with the Estβ1 amplicons appear to be at a selective disadvantage in competition with those carrying the Estα21\Estβ21 amplicons. To test the hypothesis that this is because Estβ1 is less able to bind insecticide than the common amplified esterases, Estβ12 was purified from the multi-resistant Habana strain of Culex quinquefasciatus, from Cuba. In its native form Estβ1 is a monomeric enzyme of 66 kDa, with a pI of 4.8. The bimolecular rate constants for interaction of Estβ12 with several OP insecticides were similar to those for the commonly elevated esterases Estα21 and Estβ21, and much higher than for the electrophoretically identical non-elevated Estβ13 and Estα3. Hence the apparent selective advantage of the Estα21\Estβ21 amplicon is not due to its greater efficiency of insecticide binding, as OP insecticides are significantly better inhibitors of all the amplified esterases than of their non-amplified counterparts and therefore should be equally effective at conferring resistance.