The acute toxicity of (confidential) to the rainbow trout, Oncorhynchus mykiss, was investigated during a dispersion method range finding study. The test, which was designed to establish the approximate toxicity of the test substance, was performed from November 11 to 15, 1996. The test was performed at 12 + or - 10C under static conditions with a control and five concentrations of test substance. Nominal concentrations of (confidential) were used for all calculations. The test substance, which was supplied by the sponsor, was combined with dilution water to formulate test media. Each test vessel was equipped with a cylinder (approximately 8 cm diameter) with screened openings at the water surface and approximately 2 cm above the bottom. A three-bladed plastic propeller inside this cylinder was used to continuously mix the test substance and dilution water and create a vortex that extended 40-50% of the distance from the water surface to the bottom of the test vessel. At the start of the test, insoluble material (a layer of white foam) was observed in vessels containing 33, 330, and 3,300 mg/L. The amount of insoluble material increased with the concentration of test substance. Juvenile rainbow trout procured from a commercial supplier were used to initiate the test. At the end of the test, control fish had an average wet weight of 0.51 g and an average length of 41 mm.