In the mosquito Culex pipiens, over-production of esterases is a common insecticide resistance mechanism. Different alleles at the esterase A and B loci are known to occur in natural populations, as shown by enzyme electrophoresis and RFLP studies on the esterase B locus. Here we analyse the variability of the esterase A locus at the nucleotide level in mosquitoes possessing or lacking over-produced esterase A. A surprisingly high level of nucleotide polymorphism is found in coding and noncoding regions, and the extent of polymorphism detected is higher than that previously described for the most polymorphic loci in Drosophila. We also show that eight strains from different localities (Africa, America, Asia) that possess the over-produced esterase A2 share the same nucleotide sequence at the esterase A locus, strongly supporting the evolutionary scenario of a unique event of amplification of this gene followed by dispersal through migration rather than the occurrence of multiple independent mutational events of the gene.