Insensitive acetylcholinesterase (Ace(R)) and five over-produced esterases (A1, A2 and B2, and A4 and B4) involved in detoxification are responsible for resistance to organophosphorous insecticides (OPs) in Culex pipiens L. from the Rhone-Alpes region, where C. pipiens control is mainly accomplished with the OPs temephos and chlorpyrifos using 0.15 mg/liter doses. The strong linkage disequilibria observed between esterases Al and Est-2(0.64), esterases A4 and B4, and esterases A2 and B2 indicate that these genes were introduced in the Rhone-Alpes region. Ace(R) and esterase A1, which appeared in the south of France 3 yr before the start of mosquito control in Rhone-Alpes, had the highest frequencies. All resistant genotypes were shown to be killed by 0.15 mg/liter temephos in natural breeding sites, but not by 0.15 mg/liter chlorpyrifos. These results are discussed in relation with mosquito control strategies.