Investigations to study (1) the species composition and distribution of Green River fishes between Flaming Gorge Dam and Ouray, Utah and (2) the ecology and life history of selected native species (Colorado squafish, Ptychocheilus lucius; Colorad chub, Gila robusta; bluehead sucker, Pantosteus delphinus; and humpback sucker, Xyrauchen texanus) in Dinosaur National Monument were conducted from May, 1964, to October, 1966. A total of 23,735 fish consisting of 9 indigenous and 11 exotic species, were taken in 639 collections by electrofishing gear, gill nets, seines, and fry gear. Flaming Gorge Dam has caused a major change in the ecology of the downstream Green River by alteration of seasonal flow and water temperature patterns as far as the mouth of the Yampa River, 65 miles below the dam. As a result, native fish populations particularly in the first 26 miles below the dam, have been largely replaced by introduced rainbow and brown trout (Salmo gairdneri and S. trutta). Below the Yampa River mouth, fish populations were similar to those reported here during the pre-impoundment years. Age and growth determinations were made from scales from 167 Colorado squawfish and 333 Colorado chubs. Both species grew slower in the years after dam closure (1963-1965) than before (1955-1962). The bonytail form of the Colorado chub grew slightly faster than the roundtail form. Length-frequency analyses of young Colorado squawfish, Colorado chubs, and bluehead suckers described seasonal growth of the first three year classes and provided evidence that these species reproduced successfully in Dinosaur National Monument every year since impoundment. During years of high summer discharge from the dam with resultant lower water temperatures (1964 and 1966), no reproduction of any native fishes was found above the mouth of the Yampa River. No juvenile humpback suckers were collected during the study. The roundtail and bonytail forms of the Colorado chub had significantly different length-weight relationships. Squawfish over 200 mm total length were entirely piscivorous, while shorter squawfish consumed microcrustaceans and aquatic insects. The diet of the Colorado chub consisted largely of aquatic and terrestrial insects.