Bioassays were made on ten organic phosphorus compounds, which are now in use or proposed for use as insecticides, to determine their comparative toxicity to fish. There was a wide range in toxicity of the different compounds with 96 hour TLm(median tolerance limit) values ranging from 0.20 to 121 p.p.m. No relationship was apparent between toxicity and chemical structure. With one exception dilution waters with different characteristics (pH, alkalinity, and hardness) apparently had little or no effect on toxicity. Bluegills were somewhat more sensitive to several of these compounds than fathead minnows. The organic phosphorus compounds were less toxic to fish than many of the chlorinated hydrocarbon insecticides. No definite correlation could be established in the toxicity of organic phosphorus compounds between fish, insects and mammals.