Woodsia obtusa comprises two cytotypes that are treated here as subspecies because they show subtle morphologic and ecological distinctions and tend to have different distributions. Tetraploid populations (subsp. obtusa ) are found throughout the eastern flora, commonly occurring on limestone. The diploid (subsp. occidentalis ) is found near the western edge of the species range, usually on sandstone and granitic substrates. Isozyme studies suggest that subsp. obtusa may have been derived from subsp. occidentalis through autopolyploidy (M. D. Windham 1993). The westernmost collections of Woodsia obtusa (all subsp. occidentalis ) come from the Wichita Mountains of Oklahoma and the Edwards Plateau of Texas. Reports of this species from the trans-Pecos region of western Texas are apparently based on misidentifications.