A genetic model describing the interaction between the wheat plant,Triticum aestivumL., and the Hessian fly,Mayetiola destructor(Say), is presented. The model is based on multiplicative interlocus interaction and gene-for-gene interaction between host and insect. It can be used to predict changes in allelic frequency in the insect and in the level of potential crop damage caused by deployment of host resistance genes, given genotypic fitnesses and initial allelic frequencies. Given reliable estimates of genetic parameters, the model can be used to make predictions where there are unequal genetic effects and frequencies and a much wider range of resistance-gene deployment patterns.