Potterˈs Lake, of 113 acres surface area, was treated with 1,100 pounds of derris powder containing 5 per cent rotenone on July 24, 1939. The resulting concentration, if even distribution was secured, was slightly in excess of 0.5 p.p.m. derris, or 0.025 p.p.m. rotenone. The distribution was made by placing the derris powder in coarse gunny sacks and towing these behind row boats. The powder forms a cake in water and a gentle kneading of the sacks was required to insure proper dispersion. Algae, rooted aquatic vegetation, rotifers, Hydracarina and most immature aquatic insects survived. Planktonic microcrustaceans, Chaoborus larvae, a snail, Campeloma decisum, and one species of leech were killed. The microcrustaceans and Chaoborus larvae were again plentiful in the spring of 1940. About 25,700 fish, large and small, were killed. The following species were involved: Anguilla rostrata, Catostomus commersonnii, Ameiurus nebulosus, Esox niger, Micropterus dolomieu, Lepomis gibbosus, Morone americana and Perca flavescens. A few fry of Catostomus, Esox and Perca were present in the lake in 1940, indicating an upward migration around a barrier in the outlet at flood time or a survival in the lake.