Population surveys of Culex pipiens quinquefasciatus carried out in 1991 and 1999 were compared with data collected in 1990 before the beginning of the control program against this mosquito. Larval samples collected in 1999 displayed resistance to the four tested insecticides: permethrin, propoxur, temephos, and chlorpyrifos. Temephos resistance ratio at LC (RR) ranged between 8.1- and 42-fold compared with 2.9- and 4.6-fold in 1990, and chlorpyrifos RR ranged between 8.6- and 123-fold compared with 6.4- and 19-fold in 1990. This increased resistance to organophosphorus insecticides was associated with a sharp decrease of susceptible genotypes at two loci (Ester and ace-1), as well as to an allele replacement at the Ester locus.