The strategies proposed for delaying the development of resistance to the Bacillus thuringiens is toxins produced by transgenic maize require high levels of gene flow between individuals feeding on transgenic and refuge plants. The European corn borer Ostrinia nubilalis (Hubner) may be found on several host plants, which may act as natural refuges. The genetic variability of samples collected on sagebrush (Artemisia sp.), hop (Humulus lupulus L.) and maize (Zea mays L.) was studied by comparing the allozyme frequencies for six polymorphic loci. We found a high level of gene flow within and between samples collected on the same host plant. The level of gene flow between the sagebrush and hop insect samples appeared to be sufficiently high for these populations to be considered a single genetic panmictic unit. Conversely, the samples collected on maize were genetically different from those collected on sagebrush and hop. Three of the six loci considered displayed greater between–host–plant than within–host–plant differentiation in comparisons of the group of samples collected on sagebrush or hop with the group of samples collected on maize. This indicates that either there is genetic isolation of the insects feeding on maize or that there is host–plant divergent selection at these three loci or at linked loci. These results have important implications for the potential sustainability of transgenic insecticidal maize.