Andropogon gerardii populations are comprised of two (or more) polyploid forms over much of the range of the species. To understand the impact of intraspecific polyploidy, polyploid cytotypes were compared over 4 y in native grasslands in Boulder, Colorado. Boulder A. gerardii populations averaged 59.6% hexaploids (60 chromosomes), 35.3% enneaploids (90 chromosomes) and 5.1% intermediate (aneuploid) chromosome numbers using flow cytometry to infer chromosome numbers. Neither mean clone area nor mean annual change in clone area differed significantly between ploidy levels. Hexaploid clones produced significantly more viable seeds than enneaploids. Enneaploids are not replacing themselves, whether that is measured absolutely or relative to hexaploids. Enneaploid reproductive effort was greater than hexaploid reproductive effort in some years and they produce substantial numbers of good seeds, but those seeds rarely have enneaploid cytotypes. The populations should eventually become entirely hexaploid. In the current populations, however, enneaploids are big, vigorous and fertile individuals.