This study was conducted to evaluate the potential for toxicity and bioconcentration following exposure of rainbow trout to methyl disiloxane fluid. Groups of twelve and 10 rainbow trout were exposed to water concentrations of 0 or 94.6 ppb. The fish were observed daily for mortality and adverse effects. Special attention was given to signs of narcosis. The fish were weighed, sacrificed, and the tissues pooled following the first death and the tissues analyzed. Slight to moderate signs of narcosis were noted after the first 24 hours of exposure. No deaths were observed until day 9, when most of the trout appeared stressed. A decrease in body weight was observed, but was attributed to decreased feeding during exposure. Small amounts of methyl disiloxane, 0.78 to 1.92 ppm, were found in the control fish, while body burdens of exposed fish were 25 to 71 ppm.