The pondweeds, Potamogeton (Potamogetonaceae), display heterophylly, both within and among species. In addition, some species display seasonal heterophylly. Consequently, the genus has long been considered to be taxonomically difficult. Potamogeton, sensu stricto, has conveniently been divided into two groups, broad-leaved species and narrow-leaved species. Phylogenetic analysis of 5S nuclear ribosomal non-transcribed spacer (5S-NTS) sequences supports the monophyly of two major clades, one essentially those previously considered to be broad-leaved species and the other those previously considered to be narrow-leaved species. Within each main clade, species groups are recognized, mostly along the lines of previously accepted subsections. For example, Potamogeton epihydrus and P. tennesseensis form a clade sister to the P. spirillus, P. bicupulatus, and P. diversifolius clade. These two subclades are in turn sister to the species previously recognized as the narrow-leaved pondweeds.

Key words: Heterophylly, pondweeds, Potamogeton, Potamogetonaceae