Echinodorus is the second largest genus in the aquatic plant family Alismataceae. The genus is naturally distributed in the New World, but many species are known world-wide as popular aquarium plants. The views upon species delimitation and infrageneric classification of the genus have been controversial. Phenotypic plasticity of aquatic plants combined with reduced and presumably convergent morphological structures pose serious problems to classification, emphasizing the need for molecular-level data. A simultaneous cladistic analysis of molecular and morphological data was conducted to resolve the phylogeny of the genus. The results showed Echinodorus (as it is currently circumscribed) to be polyphyletic. None of the currently proposed infrageneric classifications of the genus were supported in the light of phylogenetic evidence. Also, many species and subspecies level rankings were found to be unnatural. Monophyly of Echinodorus is ascertained by separating Helanthium and the monotypic genus Albidella from Echinodorus. As a result, two new combinations (Helanthium bolivianum and H. zombiense) are made, and a detailed description of the genus Helanthium is provided.© The Willi Hennig Society 2007.