A revision of Podostemum (Podostemaceae) is presented based on morphological analyses of extensive field collections. A. Michaux established the genus Podostemum in 1803 with the description of P. ceratophyllum from eastern North America. Numerous species have subsequently been described from Mexico and southern South America. (Several Indian species had previously been included in Podostemum, although they are now placed in the Indian Zeylanidium. Podostemum is a New World genus.) Seventeen specific and subspecific taxa were recognized by P. Van Royen (1954) in Podostemum as part of his treatment of New World Podostemaceae. We interpret nine species and varieties recognized by Van Royen as environmental forms of the seven species recognized in our treatment: P. ceratophyllum Michx., P. müllerii Warming, P. comatum Hicken, P. distichum (v. Chamiso) Weddell, P. irgangii Philbrick & Novelo, P. ricciiforme Liebmann, P. fruticulosum (Tulasne & Weddell) Weddell. We recognize species based primarily on stipule form (number of lobes, orientation) and to a lesser degree on leaf structure. Podostemum distichum is highly polymorphic in leaf and stipule form; five previously recognized species are interpreted as environmental forms of P. distichum. Phylogenetic analyses based on morphological characters indicate that Podostemum is paraphyletic. P. mülleri is monophyletic with species of Crenias. In contrast, ITS sequence data indicates a different placement of P. mülleri. Morphological data also place P. ceratophyllum and P. comatum as unresolved at a basal location in the topology, while P. distichumand the newly described P. irgangii are sister-species.

Key words: phylogeny, Podostemaceae, Podostemum, riverweeds, systematics, taxonomy