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Trichocereus (Cactaceae: Cactoideae: Trichocereeae) is a South American genus primarily inhabiting arid and semiarid areas in the Andean region of Ecuador, Peru, Bolivia, Chile and Argentina. The phylogenetic relationships of Trichocereus were examined on the basis of 39 exomorphological characters and chloroplast DNA sequences trnL-F and rpl16 for 17 species of Trichocereus, including three other genera of the tribe Trichocereeae (Echinopsis, Lobivia, Setiechinopsis), two of Notocacteae (Eulychnia, Wigginsia) and one of the Hylocereeae (Harrisia). The simultaneous phylogenetic analysis of both morphological data and noncoding DNA sequence data recovered Trichocereus as monophyletic if two species of Harrisia are part of it. Moreover, the Trichocereus clade is supported by three synapomorphies: basitonic growth with prostrate branches, imbricate scales along the floral tube, and subglobose fruits.