Chromosome numbers for 52 populations representing eight South American species of Stemodia (Scrophulariaceae) were determined. The numbers 2n = 22 found in S. hassleriana and S. palustris, and 2n = 44 in S. lobelioides are the first records for the species, while those found in S. hyptoides (2n = 22, 44) and S. stricta (2n = 22) constitute new cytotypes for those species. The basic chromosome number X = 11 was confirmed for the New World species. Chromosome numbers indicate the existence of a polyploid series in S. hyptoides with 2n = 22, 44, 66. Moreover, the existence of at least three different ploidy levels, both within and among species, indicates that polyploidy has been one of the mechanisms involved in the evolution of the genus. The geographical distribution of different species and cytotypes are analysed and discussed in the light of their extant morphological variation and taxonomic implications.