We have investigated the tissue distribution of overproduced esterases A (A1 and A2) and B (B1 and B2) in strains of Culex pipiens L. by immunocytochemistry. S-LAB mosquitoes, lacking overproduced esterases, were used as reference. Tissues showing a strong specific reaction (fluorescence) were observed with anti-esterase A1 antiserum in S54 (with A1) and BOUAKE (with A2) strains, and with anti-esterase B1 antiserum in TEM-R and EDIT (with B1) and BOUAKE (with B2) strains. Overproduction of esterases A and B was tissue-specific. The most constant pattern for the two types of esterases was their overproduction in the alimentary canal and Malpighian tubes, although fluorescence varied in intensity depending on strains and developmental stages. There was no difference in the tissue distribution of esterases A1 and A2. In contrast, esterases B pattern was highly variable among strains. Differences between TEM-R and EDIT were explained by the different overall overproduction and number of copies of the amplified gene (10-fold higher in TEM-R). The most striking difference in esterase B1 and B2 tissue localization concerned the nervous system where neurons were intensely fluorescent in TEM-R and EDIT (B1), but not in BOUAKE (B2). All esterase B positive tissues in TEM-R contained large quantities of esterase B1 mRNA (in situ hybridization), indicating that at least part of the protein revealed by immunochemistry was produced in the tissues where it was observed. Our results are discussed in terms of the protection that the different esterases can confer during exposition to organophosphorous insecticides.