The diversity of fish species in an transition zone between the River Paranapanema and the head mouth of the Jurumirim Reservoir was evaluated. Fish samples were collected from October 1995 to September 1996, by experimental fishing (gill‐nets and sieve), in three marginal shallow lagoons and two channel river zones. Gill‐net collections resulted in 21 taxons of native fishes. Detritivorous Characiformes such asSteindachnerina insculptawere predominant. Sieve collections resulted in 31 taxa and the Tetragonopterinae subfamily was the/most representative group, which included small species asCheirodon stenodon.The equi‐tability index characterized the high heterogeneity of fish numeric abundance, reflecting the dominance of a few groups. A significant relationship between biotic and abiotic variables was indicated by the first canonical function. In comparison with other regions of the drainage basin, the ecotone zone studied indicated a higher species richness and, in the case of some species, fishes in all stages of development. This study demonstrated the necessity to preserve the ecological zone for the fishing and fishery management in the reservoir.