The role of natural selection in molecular evolution has been inferred primarily by rejection of null hypotheses based on neutral theory, rather than by acceptance of specific predictions based on selection. In this report, a population genetic test of a specific prediction for selection on DNA polymorphism is presented. Pyrethroid insecticide use constitutes an experiment for which form of selection and molecular target (voltage-gated sodium channels) are both known. As predicted, differential pyrethroid selection on tobacco budworm populations generated significant geographic heterogeneity in sodium channel marker allele frequencies, compared with arbitrary loci.