To determine the levels of malathion resistance and prevalence of the malathion carboxylesterase mechanism among mosquitoes in Sri Lanka.Bioassays were carried out using WHO-recommended methods on samples of the following Sri Lankan mosquito vectors: Culex quinquefasciatus, C. tritaeniorhynchus, C gelidus, Anopheles culicifacies B, A. subpictus, Aedes aegypti and A. albopictus. FINDINGS Malathion-specific carboxylesterase mechanisms were found in A. culicifaies and A. subpictus, both giving high rates of insecticide metabolism. In contrast, malathion resistance in C. quinquefasciatus and C. tritaeniorhynchus is linked to broad-spectrum resistance to organophosphorus compounds due to elevated levels of esterases that sequester malaoxon, but are unable to metabolize malathion.Resistance among the Anophelesspp. must have occurred as a direct result of antimalarial activities, since malathion use in Sri Lanka is limited to public health treatments. In contrast, resistance among Culex spp. has resulted from large-scale use of the organophosphorus insecticide group as larvicides for filariasis control and on rice paddy, where C tritaeniorhynchus predominantly breeds, for agricultural purposes.