Rhexia is a relatively small genus of the Melastomataceae, a pantropical family of approximately 150 genera and 4000 species. The members of this family are, in general, readily recognized, for in addition to their one basic type of floral structure, the leaves are characteristically opposite and have a pronounced palmate-arcuate type of venation. Consequently, in those parts of the tropics where the family is well represented, it contributes a distinctive element to the flora. None of the species of this large family is known to be of any appreciable economic importance. Although a few become large trees, their sporadic and infrequent occurrence prohibits even these from being commercially important as timber. Medicinal substances are unknown in the melastomes, and edible products are limited to the fleshy fruits of only a few species. Although some are cultivated as ornamentals, none has ever become very popular or extensively planted. The family is represented in the United States only by Rhexia and a single West Indian species, Tetrazygia bicolor (Mill.) Cogn., which extends into southern Florida. The genus Rhexia is known only from the United States, except for the southern extension of Rhexia cubensis into the West Indies and the northern extension of Rhexia virginica into Canada. Although Rhexia has been the subject of several revisions during its history, the genus has never been adequately monographed. The limits and rank of certain taxa have continued to be a source of disagreement, and some species are little known or have never been evaluated since they were originally described. Many of the specimens cited herein were specifically collected for this study by R. B. Channell, D. Demaree, E. S. Ford, R. K. Godfrey, C. E. Wood, and the author. The majority of these collections (including those supporting experimental data) are preserved in the Herbaria of Duke University and the Florida Agricultural Experiment Station. However, most of the material examined in the course of this study was borrowed from the following institutions:2