Larval susceptibility to temephos, malathion and fenitrothion were studied in 6 Aedes, 2 Culex and one Anopheles species from areas with the following agricultural characteristics: VdT, intensively irrigated crops; Zone A, cereals and some irrigation; Zone B, cereals and grasslands; Zone C, oak and pine forests. The susceptibility of adult Anopheles from VdT and Zone B areas to dieldrin and DDT was also studied. The use of insecticides in these areas probably decreases from VdT, to Zone A, to Zone C, to Zone B. Tests of larval and adult susceptibility of mosquitoes in these areas followed the recommendations of WHO re materials and methods. In the absence of any baseline data concerning the susceptibility of the tested mosquito population, the test results only allow one to suspect changes in susceptibility to temephos, malathion and fenitrothion in larvae of An. atroparvus and Cx. theileri from VdT and resistance in adults of An. atroparvus to dieldrin and DDT.