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 cladogram retrieved indicates that the representatives of Mimosa sect. Batocaulon are not clustered, the xerophylous representatives of this section are basal, and the remaining species are related to those species of section Habbasia, suggesting that section Batocaulon ser. Stipellares is more recently derived. The species of Mimosa sect. Calothamnos that were analyzed are nested in section Mimosa. The results seem to support retention of this section within section Mimosa as was noted previously by Bentham. The chloroplast sequence data suggest that the representatives from sections Calothamnos and Mimosa share a common ancestor with those from section Habbasia and section Batocaulon ser. Stipellares.