The mechanism of resistance towards organophosphate insecticides in Culex complex mosquitoes has been shown to be associated with amplification of the structural esterase gene. The cloned portion of the SELAX strain A2-B2 amplicon was compared to that of the B1 amplicon which had been partially characterized. Fragments of known sizes from clones were used as probes to estimate the length of the B2 amplicon in the SELAX strain of mosquitoes. The two types of clones that were isolated and designated as N4 and N6, differed from each other by their size of inserts. Both clones hybridized with the 1.8 kb A2 probe and the 1.3 kb B1 probe; revealing that both N4 and N6 clones contained A2 and B2 esterase genes. Restriction enzyme digests of the clones suggested that the cloned portion of the B2 amplicon was 16 kb.