Technical Abstract: To access the relative potency of insecticides to control adult mosquitoes, 19 pesticides with various modes of action were evaluated against three medically important mosquito species. On the basis of 24-h LD50 values after topical application, the only pesticide that had higher toxicity than permethrin was fipronil, with toxicity higher than permethrin for 107-, 4849-, and 2- fold against Aedes aegypti, Culex quinquefasciatus Say, and Anopheles quadrimaculatus Say, respectively. Abamectin, imidacloprid, and spinosad also showed relatively high toxicities against the three mosquito species , with LD50 values lower than 0.0032 'g per mg of mosquito ('g/mg). However, bifenzate, dicofol, amitraz, and propargite all showed very low toxicities against the three species, with LD50 values larger than 0.1 'g/mg. On the basis of 24-h LD50 values, Cx. quinquefasciatus was the least susceptible species to 9 pesticides tested, whereas Ae. aegypti was the least susceptible species to 6 pesticides tested, and An. quadrimaculatus was the least susceptible species to 4 pesticides tested. Our results revealed that different species of mosquitoes had different susceptibility to pesticides, demonstrating the need to select the most efficacious compounds for particular mosquito species to achieve successful mosquito control.