Relative bioavailability (RBV) of arginine (Arg) from Indian mustard protein concentrate (IMC, 62% crude protein) and Indian mustard meal (IMM, 42% crude protein), and a commercially available soy protein concentrate (SPC, 57% crude protein) was compared with that of crystalline L-arginine (L-Arg) in rainbow trout. A basal diet highly deficient in Arg (1.23%) was formulated. Eight other isoproteic and isoenergetic diets were formulated to contain~1.35% and~1.5% Arg by adding increasing amount of IMC, IMM, SPC and L-Arg. The experimental diets were fed for 16 weeks using a standard protocol. Growth rate, weight gain (g fish À1 ) and protein (PD, g fish À1 ) and lipid (LD, g fish À1 ) deposition were increased linearly with increasing level of Arg from all ingredients. Arg availability from protein-bound sources were equal or higher than those from L-Arg. RBV of Arg from IMC, IMM and SPC were ranged from 100% and 123% than that from L-Arg (assumed as 100% bio-available). Among the ingredients, only the RBV of Arg from IMC was significantly higher than those from SPC (P < 0.05). The findings suggest that the RBV of Arg from IMC and IMM are very good and comparable to that of the commercial SPC used in this study.