The plan that guided my studies of the freshwater Fishes in the Buenos Aires province was the completion of the facts assembled by Holmberg, Berg, Perugia und Eigenmann but which had not given a thorough notion of the zoogeography and ecology of the region involved. I can now state that the southern limit of the distribution of the Fishes of the Family Characidae is the Cochicó lagoon and the Sauce Grande river, which forms the lagoon of the same name. This changes completely the notion repeatedly published by Eigenmann whose maps in the famous Princeton University Expedition Report marked the Rio Salado as the limit.