RESUMEN
Artemisia sect. Tridentatae is composed of 11 species of xerophytic shrubs, which dominate much of western North America. Sequences of the internal transcribed spacers (ITS) of nuclear ribosomal DNA were used to construct a phylogeny, examine circumscription of the section, resolve interspecific relationships, and test competing hypotheses on the origin of the section. The data support the monophyly of sect. Tridentatae, with the exclusion of A. bigelovii and A. palmeri-two historically, anomolous species. However, the ITS data provide insufficient variation to fully resolve interspecific relationships or to support major lineages within the Tridentata clade. Nuclear and chloroplast DNA phylogenies are discordant, which may be a result of interspecific gene flow and subsequent chloroplast capture, particularly related to the placement of A. filifolia and A. californica, in addition to A. bigelovii. Furthermore, the ITS data are in conflict with cpDNA data, providing equivocal evidence for competing hypotheses on the Old World vs. New World origin for the section and do not provide support for definitive subgeneric placement.