Froelichia Moench (Amaranthaceae: Gomphrenoideae) is a genus of approximately 15 species of herbs and small shrubs of dry tropical and temperate regions of the Western Hemisphere. As part of a taxonomic revision of Froelichia, a molecular phylogeny of the genus in North America has been constructed using sequences of the nuclear ribosomal internal transcribed spacer (nrITS) with weighted parsimony and maximum likelihood analysis. Sequence data has been used in conjunction with a detailed phenetic analysis to reevaluate the circumscription of the North American taxa. These analyses show that Froelichia is an evolutionarily young group which has differentiated into 8 taxa, principally in south Texas and the adjacent Chihuahuan Desert. Two distinct clades are evident, a southern, extending from north-central Mexico to Nicaragua comprised of the perennial F. interrupta (L.) Moq., and a northern including the species F. arizonica Thornber ex Standley, F. drummondii Moq., F. floridana (Nutt.) Moq., and F. gracilis (Hook.) Moq. While being genetically homogeneous, F. interrupta exhibits clear ecotypic differentiation, particularly west of the Sierra Madre Occidental. Species within the northern group are morphologically distinct but show little genetic differentiation, the result of a rapid radiation extending initially from northern Mexico and southern Texas through the Chihuahuan desert, the southern Great Plains and Gulf Coastal Plain. Additionally, molecular data provide strong support for two newly described taxa, F. xantii R. McCauley, a perennial herb endemic to the Cape Region of Baja California, and F. floridana var. latifolia R. McCauley, from the Texas Gulf Coast. Support is also provided for segregation of the south Texas endemic F. drummondii from the widespread F. floridana and transfer of the perennial F. interrupta var. cordata Uline & Bray from south Texas through Veracruz to F. floridana.

Key words: Amaranthaceae, Froelichia, ITS