The insecticide resistance mechanisms present in the aphids Myzus persicae and Aphis gossypii have been characterised and sensitive biochemical assays have been developed to monitor their presence in individual aphids. It was found that enhanced esterase activity is present in both aphid species, and that this enhanced activity results from the presence of larger amounts of the same enzyme rather than the presence of a more efficient enzyme. In Myzus persicae this mechanism alone is sufficient to confer high levels of insecticide resistance. In Aphis gossypii, it appears that the presence of insensitive acetylcholinesterase (AChE) is necessary for measurable levels of resistance to occur. During the course of this work, insensitive AChE was detected in Myzus persicae for the first time. This additional resistance mechanism, when combined with the enhanced esterase activity, was found to confer extremely high levels of tolerance against specific insecticides. This additional mechanism is rare in the UK at present but the use of sensitive assays to monitor its existence is of increasing importance. When the AChE mechanism becomes more prevalent in the UK, as it almost certainly will, new strategies for aphid control will be needed. Further examples of insensitive AChE conferring insensitivity not only to carbamates, but also to organophosphates, have also been detected in Aphis gossypii. The inter-relationships of the two mechanisms in this species have been resolved and new monitoring methods made available.