This study was undertaken to determine the effects of rotenone applied during a fish control operation in September 1962 and the installation of Flaming Gorge Dam in November 1962 upon the distribution of invertebrates in the Green River. Since these two events, the river has changed from a warm, turbid stream to a cold, clear trout stream for about 45 miles below the dam. Totals of 234 bottom samples and 394 drift samples were collected between the dam and Ouray, Utah (166 miles below the dam). The species composition of the fauna above Carr Ranch was much simpler during 1964-65 than the reported pre-impoundment composition. Below Carr Ranch the species composition of the invertebrate fauna has changed little. Bottomfauna densities were highest near the dam (max. 6347/ft. 2) and decreased with increasing distance below the dam. Population densities below Carr Ranch (42.7 miles below the dam) appeared to be similar to reported pre-impoundment densities. Drift rates of Baetis nymphs and Simuliidae larvae were highest near the dam. Illumination, population density. of other organisms, and water temperature had significant effects on drift-net catches of Baetis and Simuliidae. Turbidity and water - level fluctuations had important effects under certain circumstances, while date, dissolved oxygen, and depth of water had little effect on drift - net catches.