Juvenile rainbow trout (Salmo gairdneri) reared for 12 wk on different levels of dietary fiber showed a significant growth depression at levels of 10 and 20% alpha-floc (α-cellulose). The trout adapted to increased dietary fiber by increasing feed consumption, gastric evacuation time, and possibly by increasing the stomach volume and/or distensibility. The dry matter digestion coefficients of the diets declined as the fiber level increased, indicating that the digestibility of α-cellulose was not significantly different from zero. No effect of increased dietary fiber (alpha-floc) was detected on the liver glycogen levels, liver–body weight ratios, hemoglobin, hematocrit, plasma glucose, and plasma protein levels or the kidney, liver, and carcass levels of copper, iron, and zinc. We conclude that dietary fiber levels for rainbow trout should be less than 10% of the diet.Key words: rainbow trout, dietary fiber, maximum fiber level, fiber digestibility