Material collected from the fishes from the dams and fish farms of the Departamento Nacional de Obras contra as Secas (DNOCS) and from the tanks of the Itaipu Hydroelectric Station was examined in order to analyze the index and the infestation of Monogenea. Specimens of tambaqui (Colossoma macropomum), pacu (Piaractus mesopotamicus), pirapitinga (Piaractus brachypomus) and the hybrid tambacu (Colossoma macropomum X Piaractus mesopotamicus) were examined and five species of Monogenea were identified including: Mymarothecium viatorum Boeger, Piasecki & Sobecka, 2002, Mymarothecium boegeri Cohen & Kohn, 2005, Notozothecium euzeti Kritsky, Boeger & Jégu, 1996, Notozothecium januachensis Belmont-Jegu, Domingues & Martins, 2004 and Anacanthorus penilabiatus Boeger, Husak & Martins, 1995. Infection prevalence and intensity of infection and new data concerning Monogenea are presented. Key words: Monogenea, Parasites, Brazil, freshwater fishes.