AbstractThe susceptibility of three clones ofAphis gossypiiGlover to 15 insecticides was established by bioassay. A high level of resistance towards pirimicarb was confirmed for a clone from Holland (Dutch R) and a clone from Japan (Jap R), while the susceptible clone (S) was killed by very low doses of the insecticide. However, only limited cross‐resistance was shown towards other carbamates and organophosphates, and no marked resistance to the pyrethroids tested. The acetylcholinesterase (AChE) of both resistant clones hydrolysed acetyl‐choline faster than that of susceptible aphids, with greatest enzyme activity shown by the Dutch R clone. Inter‐clone differences in these rates were consistent with differences in catalytic centre activities. Inhibition (I50) of AChE by pirimicarb was approximately 900‐fold higher for the resistant clones than for the S clone. First‐order kinetics revealed that resistance to pirimicarb in Dutch R and Jap R involved a modified AChE which had a reduced (approximately 350‐fold) affinity (Kd) for pirimicarb. The marked change in AChE affinity for pirimicarb was not repeated with the other carbamates tested, ethiofencarb and aldicarb. It was considered that the resistant aphids would not require mechanisms in addition to insensitive AChE in order to show the high level of resistance to pirimicarb shown in t