The first ichthyologic inventory was carried out at the Balsas river middle basin within Darien National Park, Republic of Panama in order to determine the freshwater fish population composition and distribution. To accomplish this, intensive collections were done with the aid of fishing, dip and cast nets at 12 stations and nine riverine systems during September-October 1997. A total of 511 freshwater fish individuals belonging to 40 species, 38 genera and 17 families were collected or observed. The 79 % of total fish captured belonged to the primary fishes (including 12 characins and seven armored catfishes) totaling 405 individuals. Nine secondary fishes were found, mainly cichlids and guppies, as well as four peripheral fishes. Most abundant and representative species collected where Roeboides occidentalis, Astyanax fasciatus, Aequidens coeruleopunctatus, Gephyrocharax atracaudatus and Creagutus affinis. A total of 11 freshwater fish were considered as endemic species where found at the study area, including Brycon argenteus, Gephyrocharax atracaudatus, Hemibrycon dariensis, Pseudocheirodon arnoldi, Lasiancistrus planiceps, Rineloricaria altipinnis, Sturisomatichthys citurensis, Amphilophus calobrensis, Geophagus crassilabris, Vieja tuyrensis and Rivulus chucunaque. Thirty-eight species were recorded for the first time at the Balsas river basin, including the confirmation of the presence of Pseudophallus starksii.