The cause of high resistance to chlorpyrifos observed in Tunisian Culex pipiens (L.) was investigated by comparing a Tunisian strain G (Ͼ10,000-fold resistance), a French strain T (Ϸ50-fold resistance), and a susceptible reference strain S. Strains G and T had the same level of propoxur resistance (Ϸ1,000-fold) and were homozygous for an autosomal propoxur-insensitive acetylcholinesterase (AChE-1). In G and T strains, as well as in the offspring of different F 1 s and backcrosses using these F 1 s and the S strain, the effect of DEF and Pb synergists on chlorpyrifos resistance was low or absent, indicating that increased detoxiÞcation by enzymes inhibited by these chemicals had a minor role. Chlorpyrifos resistance in the G strain was caused by a major gene (or group of genes) tightly linked to the Ace-1 gene (coding AChE-1 enzyme). The possibility of allelism between this gene and the Ace-1 R allele present in the T strain was rejected by showing that AChE-1 inhibition by chlorpyrifos-oxon was not different between G and T mosquitoes.