| Abstract: Samples were taken from November 1983 to October 1989 in sites along the main body of the Itaipu reservoir and its area of influence to evaluate the impacts of this reservoir on ichthyofauna and to provide information about the colonization process in the dammed area. The ichthyofaunistic similarity between the sites was analysed during the studied period. Spatial and temporal variations of the ichthyofaunistic diversity were also done, using Shannon-Winer index of Diversity, eveness and species richness, comparing the results with those obtained before the damming. Afler Itaipu Reservoir formation, it was observed significative alteration in specific abundance and ichthyotauna structure of the Parana River, with an increase in species number in all tributaries of the reservoir, and a high replacement in species. Diversity and species richness were higher in the dammed stretch of the Parana river and in its tributaries ofthe lefl side afler the Itaipu reservoir formation. Among the sampled sites, the highest values of the diversity index were registered in Parana River upstream and downstream the reservoir, Iguatemi river and Arroio Guacu. Based on information of similarity in species composition and abundance in the sampled area, five site groups could be identified. |