One of the important features of a research program on blackfly control in Alaska in 1947 was an investigation of the effects of several recently developed insecticides on fish inhabiting blackfly streams. Trough and stream experiments were conducted, with rainbow trout as the principal test species and utilizing also Dolly Varden trout, king salmon, silver salmon, and red salmon. DDT, benzene hexachloride, chlordan, and chlorinated camphene were the chief toxicants tested and were applied as xylene-Triton emulsions, acetone solutions, and, in some experiments, as fuel-oil solutions and Velsicol AR-50 solutions. Blackfly larvae and other stream-bottom organisms were tested with the fish in both trough tests and in stream treatments. The minimum lethal concentrations for most of the chemicals were determined in the troughs for the three groups of organisms, and margins of safety between blackfly larvae and fish and fish-food animals were tested in the stream.