Some aspects of morphology, taxonomy and nomenclature about Monvillea phatnosperma (K. Schum.) Britton & Rose (Cactaceae), a species native from the north of the 'Great Chaco' and adjacent areas, are considered. M. phatnosperma var. arenasii var. nov. is described and ilustrated in replacement of Monvillea kroenleinii R. Kiesling -nom. inval.