The mosquito Culex pipiens is subjected to organophosphate treatments in both Corsica and southern Sardinia, but the resistance gene A2-B2, which is currently in a worldwide expansion, has only reached Sardinia. In order to understand this situation, the genetic structure of populations sampled in Sardinia and Corsica was assessed using 15 isozymes. Two loci (HK1 and HK2) were not taken into account because of the possibility of selection. For the other loci, statistical independence was not rejected for all possible pairs, and no deviation from Hardy-Weinberg expectations was apparent. Low but significant genic differentiation was present between Corsica and Sardinia, as well as between northern and southern Sardinia, despite a large number of effective migrants per generation. These results are discussed in the context of the high probability of extinction:recolonization of breeding sites, the flight migration ability of this mosquito, and the pleiotropic cost of insecticide resistances genes. It is concluded that A2-B2 resistance is unlikely to reach Corsica from southern Sardinia, unless accidental human transportation occurs.