The nutritional suitability of some Bangladeshi oilseed by-products (mustard, Brassica juncea; linseed, Linum usitatissimum; sesame, Sesamum indicum) as fish meal substitutes in carp diets was investigated. These protein sources were shown to cause depressed growth and feed efficiency when substituting 25% or more of the fish meal protein in semi-purified diets. However, the use of these oilseed meals in combination was found to be more effective than that of single sources. Supplementation of plant protein diets with crystalline EAA improved their nutritive value. Growth performance was better in fish fed diets supplemented with all deficient EAA than in fish fed diets supplemented with the first limiting EAA. Nutrient digestibility studies with these plant proteins suggested reasonable agreement between apparent protein digestibility (APD) and average apparent amino acid digestibility (AAAD). APD and AAAD values ranged from 78.9% to 85% and 82.4% to 85.8% respectively. Both aqueous and enzyme treatments were effective in reducing (49% and 57% respectively) the anti-nutritional factors (e. g. allyl isothiocyanate) in mustard oilcake. In linseed and sesame meals heat treatment was the most effective (reducing phytic acid levels by 72% and 74% respectively). Use of detoxified meals in diets improved growth performance and food utilization compared to untreated meals. Dietary phytic acid in the presence of increased levels of calcium and magnesium significantly (p < 0.05) depressed growth, food utilization and mineral bioavailability (especially Ca and Zn) in carp. Carp were shown to be tolerant of a dietary glucosinolate (allyl isothiocyante) level of 0.4 mg glucosinolate/g diet without inhibiting growth performance or adverse effects on fish health. However, fish fed diets containing higher levels of mustard oilcake or allyl isothiocyanate showed abnormal changes in thyroid tissues. The results of this study are discussed in relation to previously published research on fish and other monogastric animals.