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. DDT in acetone solution and fuel‐oil solution, chlordan in acetone solution and fuel‐oil solution, and toxaphene in acetone solution showed some promise as effective blackfly toxicants likely to be safe for fish and fish foods. Stomach‐content analyses showed blackfly larvae to represent 1.8 percent by weight of the total contents of trout and salmon examined, and their loss as fish food is not considered as serious in the area studied.