This study was designed to evaluate the aquatic effects of Dimethylcyclosiloxane fluid. Rainbow trout were exposed to dechlorinated municipal water only or water containing approximately 2.5 ppb of the test substance for 14 days. Mortality, narcosis, and any other signs of toxicity were checked daily. Following exposure, the fish were homogenized in distilled water and analyzed by solvent extraction (hexane) and gas chromatography/mass spectroscopy analysis. Water from the exposure chambers was also analyzed by gas chromatography/mass spectroscopy for the test substance. The amount of material present was quantified by comparing the total area counts of the unknown versus that of the internal standard. Dissolved oxygen levels and pH were determined 3 times per week. The control water contained levels of the test substance that at times almost reached that of the treated water of 2.4 ppb. This was thought to be the result of contamination from volatilization of The test substance from the discharge pipe. The mean amount of D5 found in the fish was 5500 ppb (or 0.015 mM of D5/gm wet wt.). The bioconcentration factor in the fish is from a single data point but can be assumed to be at least 2000. There were no mortalities among any of the exposed fish.