The Rio Manso, a tributary to Rio Cuiabá, Rio Paraguai basin, is situated upstream from the Brazilian Pantanal and its ichthyofauna has many species in common with the typical fishes from the Pantanal and with those from the Rio Paraguai proper. The Manso Reservoir region of influence was sampled up to the Pantanal, including different environments, such as rivers, streams, lagoons, and the Reservoir itself, in the Rio Manso. Fish species were collected from March 2000 to February 2005, using different fishing gear, such as gill nets, seines, casting nets, and electro fishing. Considering the entire period, 250 fish species were collected, and are distributed in three Classes, 11 Orders, and 37 Families, from which 33 species do not yet correspond to any formal description, indicating that they should be new.