Carob seed germ meal (CSGM), and soybean meal (SBM), were tested as ingredients of diets for rainbow trout (Oncorhynchus mykiss) fingerlings at three levels of inclusion, contributing 20 to 50% of the total protein of the diet. Fish growth decreased as the level of the plant by-product in the diet increased. Diets containing CSGM resulted in lower growth than comparable SBM diets, especially at inclusion levels contributing 37 and 53% of the total protein. High dietary CSGM levels decreased body protein and increased liver protein indicating possible amino acid deficiencies and/or imbalances at high inclusion levels. Mortality was also high at the high dietary CSGM level. The results are discussed with référence to the antinutritional factors contained in the raw materials tested and their possible effects on protein availability.