A marked difference between some Characiopsis species from Northeastern Argentina and those from the literature of the rest of the world was noticed when observing natural populations. This induced us to make a characterization of the genus Characiopsis Borzi (Mischococcales, Tribophyceae) from the taxo? nomic, biogeographical and ecological points of View. The taxonomy of the group was completed on the basis of the compilation made by Ettl (1978), plus the? taxa added other authors afterwards, and our observations on natural populations from a variety of environments and substrata of the Argentinian Northeast. Besides 11 species and 22 varieties new to science, 2 new records for Argentina and the Southern Hemisphere were described. Eight complexes (If taxa exhibiting a close morphological affinity among them, thus being a continuum, were also erected. A study of basic characters used for taxonomic assigments was performed by means of a cluster analysis, in order to assess the variability pattern of the genus. Finally, the genus was characterized from both ecological and biogeographical points of view, stressing preferences of substrata and type of environment. The world distribution is also presented.weiterlesen