A sensitive technique allowing to identify the three genotypes (AceSS, AceRR and AceRS) of the Ace gene existing in natural populations of Culex pipiens in southern France is described. The technique is based on the comparison of AChE (acetylcholinesterase) activity in 3 equal aliquots taken from the homogenate of a single mosquito (a) in absence of inhibitor (RA), (b) in presence of eserine that inhibits the AChE encoded by AceS and AceR alleles (RI) and (c) in presence of a concentration of propoxur inhibiting the AChE coded by the AceS allele but not by the AceR allele (RG). The mosquito tested is AceSS when RG = RI, AceRR when RG = RA and AceRS when RI less than RG less than RA.