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INTRODUCTION The 1962 rotenone treatment of the Green River in Wyoming offered an excellent opportunity to study the effects of the treatment on the fauna, both at the time of treatment and during the post-treatment recovery period. The efficiency of the treatment, its effects on fish organisms, and the post- treatment re-entry of fish species into the treated area all merited study. Ac- cordingly, studies were conducted before and after treatment to evaluate the treatment and its effects on the river fauna, especially the fish food organisms. Sampling activity was initiated in July, 1962, two months prior to treatment, and continued until September, 1964. Although some work was done on other sections of the treated area, the primary emphasis of the study was on the Green River and its principal tributary, the New Fork River, from a point shortly above the treated area; to the Flaming Gorge Dam, about 117 miles downstream. Only selected macroinvertebrates and the fishes of the river were sampled during the study. The results of the study are presented in this report and in Binns, 1965. The rotenone treatment of the Green River followed congressional author- ization of the two main-stream dams, Flaming Gorge and Fontenelle, of the Colorado River Storage Project. Construction of these two dams presented the Wyoming and Utah Game and Fish Departments with a choice of either (1) trying to establish a trout fishery, after impoundment, in the presence of a rapidly expanding non-game fish population, or (2) depressing the non-game fish population sufficiently, before impoundment, to permit introduced game fish to become established in the new reservoirs. Since the entire area involved had an excellent potential as a future trout fishery, the decision was made to treat the river before impoundment. Cost of treatment after dam closure would have been prohibitive. Thus, about 445 miles of the Green River and its tributaries upstream from the Flaming Gorge Dam were treated in early September, 1962, with Chem-Fish Regular, an emulsified rotenone product.l The treatment operation is described in detail in a j oint report by both depart- ments (Binns, et al, 1963) ; however, a brief description of the treatment pro- cedures will be presented later in this report. The limnology of the Green River, below LaBarge, has been described in detail by Bosley (1960) and by McDonald and Dotson (1960) ; both publica- tions suggest depressing the coarse fish population with rotenone. In addition, investigations have been made of the aquatic biota of the Green River in Dino- saur National Monument, situated about 50 miles downstream from the Flaming Gorge Dam site (Utah University, 1963). iManufactured by Chemical Insecticide Corporation, Metuchen, New Jersey. Guazan- teed Analysis: Active ingredients 70 percent Methylated Naphthalene 55 Other Cube extractives 10 Rotenone 5 Inert ingredients 30 Total i00 8