Three experiments and a digestibility trial were conducted in order to assess the incorporation of extruded lupin (Lupinus albus) in diets for juvenile rainbow trout. Digestibility of protein and phosphorus were higher in lupin than in fish meal, but digestibility of dry matter and energy were lower. The first trial was designed to determine the maximum level of incorporation of lupin in the diet of trout. Levels of 30, 50 and 70% were tested and compared with a fish meal-based control diet. The diets were formulated to be isonitrogenous and isoenergetic. Triplicate groups of trout were fed twice daily to visual satiety by hand during 64 days. Two subsequent trials, using another yearly crop of lupin, were performed to analyze the effects of very high levels of incorporation of lupin. Growth performances, feed intake and nitrogen balance of fish fed diets with 50% of lupin incorporation were comparable to those of fish fed the control diet. However, higher fat deposition was observed. Incorporation of lupin led to higher phosphorus retention and lower phosphorus excretion, but only in two of the three trials. In trout fed the diets containing 70% lupin, growth was reduced by 41%, feed intake by 15% and nitrogen retention by 12% when the first crop of lupin was used. Feed intake was not reduced and growth performance was higher when the second crop of lupin was used, i.e., a decrease of only 16% when fish were fed by hand to satiety (decrease of feed efficiency) or null when fish were fed on demand using self-feeders, The incorporation of lupin can lead to a decrease in plasma thyroxine levels, hut this effect was not clear and not recurring. However, when this effect was observed, a deiodinase compensatory effect adjusted the plasma triiodothyronine levels. In general, the plasma triiodothyronine levels were related to the growth performance of the trout.