The diverse floral morphology in the Asclepiadaceae historically has created taxonomic confusion in the family, especially at the generic level. In 1954, Robert Woodson sank 20 North American genera into synonomy under Asclepias, creating nine subgenera and an additional eight series under the largest subgenus, Asclepias. To clarify relationships within the genus of approximately 114 species, we have phylogenetically analyzed ITS nucleotide sequences for 76 Asclepias species, representing all of Woodson's subgenera and series. The data set of 152 phylogenetically-informative characters supports the monophyly of Asclepias. A basal clade is present, composed of species from Series Incarnatae. All remaining species occur in its sister clade, which has little resolution within it. Thus, it appears that there are two major lineages within Asclepias, one defined by erect pedicels and cup-shaped stipitate hoods (e.g., Series Incarnatae,) while the other clade contains species with reflexed pedicels (with few exceptions) and more variable hood morphology.

Key words: Apocynaceae, Asclepias, Asclepidaceae, ITS, phylogeny