Amaranthus rudis and A. tuberculatus, the waterhemps, are an important weedy, dioecious species pair, the identification of which historically has been much confused. Morphological examination reveals a large number of pistillate intermediates that fit neither species and that staminate plants have not been well characterized. Principal components analysis of isozyme data of waterhemps collected from throughout the speciesí ranges shows that populations segregate along a single geographic continuum that does not correspond with the original morphological species designations. A single species, Amaranthus tuberculatus (Moq. ex DC.) J. D. Sauer, is proposed.

Key words: Amaranthus rudis, Amaranthus tuberculatus, dioecy, isozymes