Objective Insect susceptibility has been one of the most important aspects to be monitored in public health programs for vector control. The purpose of the study is to assess the susceptibility to chemical insecticides of Aedes aegypti larvae in both areas under vector control and no vector control. Methods World Health Organization standard bioassays for diagnostic concentration and multiple concentrations were performed in mosquito larvae collected in an area under no vector control (Campinas, SP) and an area under vector chemical control (Campo Grande, MS), in Brazil. Results Potential resistance to a diagnostic concentration of temephos (DC=0.04 ppm) was registered for an Ae. aegypti larval population collected in Campinas. Multiple concentration tests confirmed the larvae resistance, with 24.5% of them surviving at the 0.0125 ppm concentration. Bioassays with the organophosphate fenitrothion (DC=0.08ppm) and pyrethroid cypermethrin (DC=0.01 ppm) in the same population revealed their susceptibility to these agents. Bioassays carried out in all Ae. aegypti C larval population collected in Campo Grande showed their susceptibility to temephos (DC=0.04 ppm) and cypermethrin (DC=0.01 ppm). LC50 and LC95 for cypermethrin (CE25), cyfluthrin (CE5), betacyfluthrin (SC1.25) and propoxur (CE20) were determined for Ae. aegypti. Using the Rockefeller standard strain values, ratios of resistance were estimated: 2.9, 2.2, 2.4 and 1.3 for LC50 and 3.5, 2.6, 3.9 and 1.3 for LC95, respectively. Conclusion The findings reinforce the need for routinely monitoring pesticide efficacy as a very important step in vector control management programs.353232236