Etofenprox is a non-ester pyrethroid insecticide with comparable toxicity and a similar mode of action to other pyrethroids. Cross-resistance studies on mosquitoes showed no effect of carboxylesterase, elevated esterase, altered acetylcholinesterase or glutathione S-transferase-based resistance mechanisms on etofenprox toxicity, when compared to standard susceptible strains of Anopheles and Culex. Cross-resistance to etofenprox occurred in a pyrethroid-resistant strain of Culex quinquefasciatus with both oxidase and 'kdr'-like resistance mechanisms. Dose-response data for susceptible mosquito strains suggest that, in standard W.H.O. susceptibility tests of adult mosquitoes, appropriate discriminating concentrations of etofenprox for detection of resistance would be 0.1% for Culex and 0.25% for Anopheles.