ABSTRACT.A rapid method of distinguishing insecticide insensitive acetylcholinesterase (AChE) genotypes was applied to three species of mosquitoes. This relies on comparing rates of an AChE mediated reaction in the presence and absence of insecticides which are inhibitors, using a kinetic microtitre plate reader. Clearer and more rapid resolution between genotypes was achieved than with previous assays which measure the amount of product formed at a fixed end‐point. Results are presented for the Fls from crossing resistant and susceptibleAnopheles albimanusWiedemann andCulex pipiensL., for a strain ofAn.albimanuswith a translocation linking the AChE gene to the Y chromosome and for field collectedAn.nigerrimusGiles. Propoxur and malaoxon were used as inhibitors. In all three species the enzyme was more insensitive to propoxur than malaoxon. Susceptible enzymes in all species also showed higher uninhibited AChE activity than their resistant counterparts. Presentation of both inhibited and uninhibited activities side by side may be useful to identify insects likely to be misclassified due to abnormally low AChE activities. Estimated frequencies of the three resistance genotypes in field populations ofAn.nigerrimusconformed to Hardy‐Weinberg ratios. The implications of this technique for laboratory and field studies on insects are discus