Podostemaceae are restricted to tropical river-rapids and waterfalls. About half of the 49 genera and 60% of the species occur in the New World. This contribution focuses on phylogenetic relationships among New World Podostemoideae (the largest subfamily) based on parsimony analyses of morphological (vegetative and reproductive)characters. Apinagia (ca. 50 spp.), Marathrum (ca. 25 spp.), and Rhyncholacis (ca. 25 spp.) are the largest genera in the New World. Our analyses indicate that neither Apinagia nor Marathrum is monophyletic. Species of Apinagia occur in several clades. Some species of Marathrum occur closely related to those of Rhyncholacis, while others occur in different clades. One well-supported group is composed of one species each of Marathrum, Vanroyenella and Macarenia. Mourera is monophyletic. The genus Oserya is monophyletic, and occurs basal to a clade in which Ceratolacis, Devillea, Castelnavia, Podostemum and Crenias occur. Species of Podostemum and Crenias form a monophyletic group. Podostemum, however, is paraphyletic. Preliminary analyses indicate that Indian Podostemoideae arose from ancestors that are shared with the New World genera Podostemum and Crenias. Character state change will be discussed relative to the phylogenetic hypotheses. For example, the phylogenetic topology indicates that the andropodium (fused stamen filaments) and dyad pollen each evolved once.

Key words: phylogeny, Podostemaceae, Podostemoideae, riverweeds