The Iguaçu River is a large tributary of the Paraná River, with a highly endemic ichthyofauna due to the geographic isolation imposed by the Iguaçu falls, located near its mouth. Fish were collected monthly in four sampling stations along the Salto Osório Reservoir, from July 2003 to June 2005, using gill nets, casting nets, and long lines. Considering the entire period, 41 fish species were colleted, which belong to six Orders, 17 Families, and 27 Genera. From these, 24 species are considered endemic. Comparisons with other surveys conducted in the Iguaçu River are provided, in addition to comments on the fish endemism, even within the basin.