Abstract The pantropical and subpantropical genus Mimosa L. comprises more than 500 species, of which nearly 480 are reported for the American Continent. Mimosa is subdivided into five sections, four of which are represented in southern South America: Batocaulon DC., Habbasia DC., Calothamnos Barneby, and Mimosa. Previous taxonomic studies of the species from the austral region have found classification conflicts among (a) sections Batocaulon and Habbasia, (b) sections Calothamnos and Mimosa, and (c) series and subseries within section Mimosa. This paper reports a preliminary phylogenetic analysis of chloroplast nucleotide sequences of the trnL intron and the trnL-trnF intergenic spacer from 34 species of Mimosa and related genera. Key morphological characters were mapped onto the phylogenetic hypothesis and discussed. Sequence analysis indicates that the genus Mimosa is monophyletic; it is derived from Piptadenia viridiflora (Kunth) Benth. The four sections proposed by Barneby are not natural groups. The...