In many North American prairies, populations of Andropogon gerardii Vitman (Poaceae) are composed of hexaploid and enneaploid cytotypes (2n = 60, 90), with intermediates occurring occasionally. Under controlled pollination, the two common cytotypes can be crossed, producing progeny with a range of chromosome numbers. In an investigation of fertility and compatibilities of intermediate cytotypes, individuals with chromosome numbers between 60 and 90 were crossed with each other, with the 2n = 60 and 90 cytotypes, and with South American Andropogon species having 60 chromosomes. Regardless of cytotype, all A. gerardii plants had some fertility and virtually all crosses produced seeds. Cytotype is only partially predictive of fertility. Inter-specific hybrids between A. gerardii and South American hexaploid species were vigorous but sterile. Gene flow in natural A. gerardii populations of mixed cytotype probably involves plants of all cytotypes. © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society, 2003, 141, 95–103.