The structure of the fish communities of a pool and a rapid in the ribeirão Santa Bárbara is described. This stream of 5th order is a direct tributary of the rio Tietê in the system of the upper rio Paraná. Thirty three species, from nine families and three orders, were referred in the two mesohabitats: the pool with 31 species, and the rapid with 18. Orders Characiformes, Siluriformes, and Perciformes presented the greatest number of species in the two mesohabitats. The families Characidae and Loricariidae were the most specious, followed by Cichlidae in the pool, and by Parodontidae in the rapid. Most species were accidental, 17 in the pool, and 10 in the rapid, and only six were constant in the two mesohabitats. The greatest diversity and evenness occurred in the pool. The seasonal variation for both assemblages was high, with the pool having a high turnover of species that could be due to the proximity with main channel of the rio Tietê.
seasonality; spatial distribution; upper rio Paraná system; niche breadth; diversity; adventitious streams
