Twenty-nine species of fishes were collected in the middle and upper Colorado River basins in 1967-1973. The native suckers, Catostomus latipinnis and C. discobolus, were the dominant species in the study area. Introduced species outnumbered native species 19 to 10. The introduced Ictalurus punctatus and Notropis lutrensis were abundant throughout most of the upper basin. The abundance of introduced species has increased steadily since 1900 as has the introductions of new species. Four endemic species, Ptychocheilus lucius, Gila elegans, Gila cypha, and Xyrauchen texanus, are considered endangered. These rare forms reproduce in the lower Yampa River, Desolation Canyon of middle Green River, and the lower Green River in Canyon-lands National Park. The major reasons for the decline of native fishes are considered to be alterations of habitat by high dams and introductions of exotic species.