Objective To evaluate the susceptibility to chemical insecticides of Culex quinquefasciatus and Aedes aegypt larvae from areas subjected to control treatments or not. Methods Bioassays for diagnostic concentration and multiple concentration were performed for organophosphate and pyrethroid insecticides according to World Health Organization parameters. The susceptibility was assessed for mosquito larvae collected from (in area not subjected to chemical control (Campinas, State of Sao Paulo, SP) and from other areas (Campo Grande, Mato Grosso do Sill, MS, and Cuiaba, Mato Grosso, MT), in Brazil, subjected to such treatments. Results Tests for Culex quinquefasciatus larvae from Campinas, SP allowed suspicion of resistance to cypermethrin and gave evidence of resistance to cyfluthrin. Larvae of this species collected in Campo Grande, MS, and Campinas, SP presented resistance to temephos. For the colony from the latter locality, the following resistance rates were established: RR50 =6.36 and RR95 =4.94, in relation to a standard susceptible strain. Moreover, tests for Aedes aegypti showed similar susceptibility to temephos for a field population from Cuiaba, MT and a laboratory population. Conclusions The results indicate resistance of Culex quinquefasciatus to organophosphate and pyrethroid insecticides and make evident the need for evaluation and monitoring of the efficiency of insecticides to be used in mosquito control programs.374523527