Dominance level of insecticide resistance provided by one major gene (an insensitive acetylcholinester-ase) in the mosquito Culex pipiens was studied in two distinct environments. Dominance level was found to be very different between environments, varying from almost complete dominance to almost recessive when either propoxur (a carbamate insecticide) or chlorpyrifos (an organophosphorus insecticide) was used. To better understand this plastic response, three environmental parameters were manipulated and their interactions studied. For chlorpyrifos, each parameter had a small effect, but when all parameters were changed, the dominance level was greatly affected. For propoxur, one environmental parameter had a large effect by itself. It was further studied to understand the causal relationship of this plasticity. Recessivity of resistance was associated with more demanding environments. These results are discussed in the context of the various theories of the evolution of dominance. It appears that dominance of insecticide resistance cannot be directly predicted by Wright's physiological theory. T HE evolution of dominance has been the object of extensive controversy. FISHER (1928, 1931, 1958) proposed that the modification of dominance by other loci (modifier genes) was the basis of the dominance of wild-type alleles. WRIGHT severely critized this theory, showing that the selection pressure on dominance modification would only be of the order of the mutation rate. He proposed an alternative hypothesis known as WRIGHT'S physiological theory (1929, 1934, 1977). WRIGHT assumes that most loci code enzymes and that most mutations are deleterious, causing a reduction in enzymatic activity. If the wild type is more active than necessary, then the rate of reaction is likely to be sub-strate limiting rather than enzyme limiting. Thus a dele-terious allele that only slightly reduces enzyme activity should appear recessive or nearly recessive. Growing evidence against FISHER'S theory (CHARLES WORTH 1979; ORR 1991) makes the physiological theory the most appropriate. HALDANE (1930), MULLER (1932) and PLUNKETT (1933) have proposed alternative models but all are related to WRIGHT'S theory, and KACSER and BURNS (1981) and KEIGHTLEY and KACSER (1987) have given a detailed enzyme kinetic analysis of dominance. However, the physiological theory of dominance has remained mostly theoretical due to the absence of clear empirical evidence (but see ORR 1991). Insecticide-resistance genes have occurred recently in numerous insect species and have been intensively studied. Dominance levels of insecticide resistance pro-Curresponding author: Denis Bourguet, Institut des Sciences de 1'Evo-lution (URA CNRS 327), Lab. Ginitique et Environnement, Uni-versiti Montpellier I1 (C.C. 065), 34095 Montpellier cedex 5 ,