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The nature, composition, and evolution of Centra) American freshwater fish groups are discussed in relation to the history and derivation of the faunal elements. It is concluded that the entire area between the Isthmus of Tehuantepec and eastern Panama was and always had been devoid of primary (obligatory) freshwater fishes prior to the very late Tertiary. Instead, secondary freshwater fishes evolved in this area during the Neogene and perhaps for a longer period, the Poeciliidae having probably had a longer history within the area than the Cichlidae. In the late Tertiary a very few North American immigrants entered the area from the north, and towards the end of the Pliocene the closing of the very ancient Panama sea gap permitted an influx of South American primary types. Most of these have not yet gotten past Costa Rica, but a few aggressive characids have reached Guatemala or southern Mexico and one has reached Texas. It follows that the rich South American primary freshwater fish fauna could not have been originally derived from or through Central or North America, and continental drift (not here discussed in detail) is suggested to explain South American-African similarities.