The activity and electrophoretic mobility of nonspecific esterases, and the sensitivity of acetylcholinesterase (AChE) to organophosphate and carbamate insecticides, was determined for 21 strains or clones of Aphis gossypii of worldwide origin and related to patterns of insecticide resistance disclosed by diagnostic dose bioassays with demeton-S-methyl, pirimicarb, and permethrin. Results confirmed previous work showing the occurrence of two forms of mutant AChE, both conferring insensitivity to pirimicarb and representing the primary mechanism of resistance to this insecticide. One of these mutant AChE enzymes also caused resistance to demeton-S-methyl in its own right, whereas the other only conferred resistance in combination with moderate or high levels of nonspecific esterase activity. Lack of any correlation between permethrin resistance and quantitative or qualitative changes in esterases implied a distinct, as yet uncharacterised, mechanism of resistance to pyrethroids, possibly based on insensitivity in the target protein for these pesticides.