Large reservoirs are increasing in number in South America and Brazil, and are exploited by fishers. The River Piracicaba receives several pollutants discharged from industries and residences, and had its downstream portion dammed in 1962. This study is about the gillnet fishery located at the impounded portion of the River Piracicaba, and the possible influences of habitat alterations on the fish catch composition. Fish landings were recorded three days each month between October 1994 and September 1995. Data on the weight of fishes caught (kg per species) in 248 fish landings were collected. The catch of the large migratory catfishes (Siluriformes) decreased in the last 10 years, probably because of the interruption of their migratory route by the impoundment. Migratory Characiforms, on the other hand, have increased in the fishery. The river organic pollution probably favoured the spread of fishes that can adapt to low oxygen conditions. 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