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Interspecific comparisons of bioassay and biochemical data suggest two major patterns of target site resistance to carbamates and organophosphates. Pattern I resistance, which is generally more effective for carbamates, has been shown in two sub-species of mosquitoes to be due to a particular Gly-Ser mutation in the oxyanion hole within the active site of one of their two acetylcholinesterase enzymes. Intriguingly, different substitutions at the equivalent site confer organophosphate hydrolytic ability on other esterases responsible for metabolic resistance in some other species. In the case of the aphid, Myzus persicae, Pattern I resistance is due to a Ser-Phe mutation in the vicinity of the acyl pocket of acetylcholinesterase. Pattern II resistance is at least as effective for organophosphates as it is for carbamates and may even be specific to organophosphates in some cases. Molecular studies on this pattern of …