Developing genetically modified crop plants that are biologically contained could reduce significantly the potential spread of transgenes to conventional and organic crop plants and to wild or weedy relatives.Among several strategies, the hereditary mode of transmission of transgenes, whether dominant, recessive, or maternal, could play a major role in interspecific gene flow. Here we report on the gene flow between foxtail millet (Setaria italica), an autogamous crop, and its weedy relative, S. viridis, growing within or beside fields containing the three kinds of inherited herbicide resistance. Over the 6-year study, in the absence of herbicide selection, the maternal chloroplast-inherited resistance was observed at a 2 X 10-6 frequency in the weed populations. Resistant weed plants were observed 60 times as often, at 1.2 X 10-4 in the case of the nuclear recessive resistance, and 190 times as often, at 3.9 X 10-4 in the case of the dominant resistance. Because the recessive gene was not expressed in the first-generation hybrids, it should be more effective than dominant genes in reducing gene flow under normal agricultural conditions where herbicides are sprayed because interspecific hybrids cannot gain from beneficial genes.