A taxonomic study of collections previously identified as Isoetes melanospora, I. piedmontana, and I. tegetiformans was undertaken emphasizing vegetative characters. The taxonomic position of  I. piedmontana was also assessed using cytogeography and a cursory examination of the allozyme locus TPI-2. Isoetes melanospora, I. piedmontana, and I. tegetiformans can be sufficiently distinguished using characters derived from leaves and corms. Isoetes piedmontana occurs as diploid, triploid, and allotetraploid entities. Diploids in the western and eastern parts of the geographic range are genetically distinct. Allotetraploids in eastern Alabama are composed of different genomes than those in North Carolina. Isoetes piedmontana is currently interpreted as an assemblage of morphologically indistinguishable species and is undergoing further taxonomic investigation. The typification of I. piedmontana is clarified, and a key with descriptions of the granite outcrop Isoetes species of the southeastern United States Piedmont is provided.