Female Aedes aegypti mosquitoes are vectors of yellow fever, dengue and chikungunya viruses. Pre-exposure of Ae. aegypti larvae to the herbicide atrazine significantly reduced their sensitivity to the organophosphate insecticide temephos. Mosquito larvae pre-exposed for 48 h to non-lethal concentrations of atrazine ranging from 1 to 10 μg/l commonly encountered in the field, appeared slightly less sensitive to temephos than non-pre-exposed larvae. The effect of a pre-exposure to atrazine on larval tolerance to temephos did not seem to be related to an induction of detoxification processes by this herbicide.Noimportant increase in glutathione transferase, or α- and β-esterase activities was observed in pre-exposed larvae, while P450 monooxygenase activities increased.