Insecticide resistance management is of central importance to an increasing number of vector control programmes. Identification of the resistance mechanisms involved is crucial for such management, as is a knowledge of the interaction between different mechanisms and the cross-resistance spectra conferred. Effective insecticide use is based on the premise that the resistance allele frequency drops in the absence of insecticide selection pressure. Negative fitness effects associated with resistance genes are therefore important: any reversion to use of pesticides employed previously is dependant on the resistance gene(s) to these pesticides not remaining in Hardy-Weinberg equilibrium, but dropping to very low frequencies when the pesticide is removed from the environment. This study illustrates (a) use of rapid microtitre plate assays on large numbers of individual mosquitoes for identification of resistance mechanisms, and (b) accurate monitoring, in laboratory populations of the pest mosquito Culex pipiens Linnaeus, of the frequencies of the genes governing these responses to insecticide exposure.