Resumo A fauna sul-americana de peixes de água doce, comparada com a do resto do mundo, é pouco conhecida. Há necessidade de coletas imediatas em muitas regiões, antes que os peixes se tornem raros ou se extingam. A fase descritiva da fauna de peixes de água doce da América do Sul desenvolveu-se em três períodos históricos. De 1750 até aproximadamente 1866, boa parte dos grandes exemplares de interesse comercial foi descrita por zoólogos europeus. De 1866 a 1930, caracterizou-se por descrições de espécies de grande e pequeno porte, por ictiólogos tanto europeus como norte-americanos. De 1930 ao presente, foi, em parte, um período em que surgiram descrições adicionais na América do Norte, Europa e América do Sul. De 30 a 40% da fauna de peixes de água doce da América do Sul estão por ser descritos. Em termos de descrição e inventariação, o conhecimento desta fauna é comparável ao da fauna de peixes de água doce dos Estados Unidos e Canadá há cerca de 100 anos. Existem atualmente 2.500 a 3.000 espécies conhecidas de peixes de água doce na América do Sul e o número final pode chegar a 5.000. A não disponibilidade de bibliografia antiga sobre peixes, de exemplares de museu para comparações adequadas, e a falta de trabalhos faunísticos para a maior parte das áreas, são fatores que dificultam e até mesmo impossibilitam ictiólogos e biólogos de pesca no desempenho satisfatório de suas profissões na América do Sul. Muitas espécies descritas antes de 1870, precisam ser estudadas e redescritas. A maior parte dos grupos de peixes de água doce sul-americanos necessita revisões taxonómicas modernas. | Summary The fish fauna of the fresh waters of South America is relatively poorly known compared with those of the remainder of the world. Current progress in the economic growth of many areas of South America, forcing alteration of the habitat by deforestation, agriculture, and industrialization, makes it imperative that in many areas the fishes be sampled soon, before they are lost. The descriptive phase of the study of the freshwater fish fauna in South America has proceeded in three historical periods. The first period occurred from 1750 to about 1866 during which a moderate share of the larger market fishes were described by European zoologists. The second period took place from 1866 to about 1930 and was a period of rapid description of small and large species by ichthyologists both in Europe and North America. The third period, from 1930 to the present, has been in part a period of further description in North America, Europe and South America. It has also been a period in which the complexity of the problems facing systematic ichthyology in South America have been more fully understood. Although no real estimate can be made, perhaps as much as 30 to 40% of the freshwater fish fauna of South America remains to be described and our current descriptive or inventory knowledge of this fauna is similar to our knowledge of the freshwater fish fauna of the United States and Canada about 100 years ago. There are currently 2,500 to 3,000 known species in South America and the ultimate figure could rise to 5,000 species. Study of the phylogenetic relationships and zoogeography of South American freshwater fishes is in its infancy. The unavailability of old ichthyological literature, comparative museum specimens, and the lack of faunal reports for most areas are all factors making it difficult and often impossible for ichthyologists and fisheries biologists to effectively pursue their professions in South America. These same factors inhibit studies of South American fishes in countries outside South America. Many species of fishes, especially those larger food fishes described before 1870, need study and redescription so that literature suitable for simple identification may be provided to the fisheries biologist and others concerned with economically important fishes. However, in addition to this, most groups of South American freshwater fishes ate in need of modern taxonomic revision before successful research on these fishes can be performed in other disciplines