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We review patterns of reservoir colonization and aging in neotropical reservoirs, with emphasis on the Upper Paraná River Basin, Brazil. The filling stage was a period of rapid and intense transformation, an abrupt transition from a lotic to a lentic environment. This stage lasted 8-80 d, which was relatively short in comparison with reservoirs in temperate zones or other reservoirs in the Amazon basin. Release of nutrients, increases in water transparency, decreases in turbulence, and development of thermal and oxygen stratification were key factors characterizing this transition. Anoxic events were critical during this stage. During the first few days individuals of various fish species occupied the entire reservoir and water column, independent of the habitat they had inhabited in the river, or the type they would eventually inhabit in the new lake. As anoxia progressed, fish moved to the embayments and tributaries including the main river. Species that remained in the reservoir concentrated mainly in shallow, littoral areas and near the mouth of tributaries. The season of the filling stage was extremely relevant to the colonization process. The ecological transformations observed during the filling stage of neotropical reservoirs are likely more accentuated than in temperate reservoirs, because water temperature remains warm around the year. After the original upsurge, no severe changes in abiotic environment were identified, but a gradual colonization by submersed aquatic macrophytes was evident in some reservoirs, and changes in the fish communities were identified in all reservoirs. In one reservoir, diversity of macrophyte communities became …