[en] Tucurui was the first big hydroelectric built in the Brazilian Amazonia (river Tocantins, Para). About 2400 Km₂ of Tropical Rain Forest was inundated. This work attempts to describe some effects of the Tucurui dam upon fish communities of the lower Tocantins river. Surveys of fish communities on the Tocantins were carried out by INPA, before and after the impoudment. Fishes belonging to 223 species was sampled by gillnets. After the impoudment, the average of species richness of the communities was reduced in 42% and 50%, respectively downstream and upstream the dam. In the reservoir, where rapids were flooded, species specialized in this habitats virtually disappeared during and after the filling up phase. The relative contribution of the illiophagous, that dominated the samples before the dam closure, downed, while there has been a sudden widespread of fish predators species in the reservoir. These predators dominated until the third year after the impoudment. The cluster analysis provide evidences that, between the species that take advantages of the impoundment, exist a clear succession of dominance. Until the end of sampling period all the study-area remained very instable, so is difficult to foresee the future composition or structure of the communities in the area under influence of Tucurui's Hydroelectric Dam. (author). 68 refs., 46 figs., 12 tabs