The Mogi Guaçu River is one of the largest rivers in state of São Paulo, belonging to Paraná hydrographic system.A study about fish composition in the Mogi Guaçu reservoir and four oxbow lakes downstream the reservoir is showed. Atotal of 2,367 individuals from six orders, 17 families, and 46 species were collected during August 2005 to July 2006, usinggillnets, traps and hand nets. In the reservoir were found 31 species and in the oxbow lakes 24, in which Curimatidae andCharacidae were the most abundant families, respectively.