A study utilizing casein-com gluten meal based diets supplemented with crystalline amino acids was conducted to determine the quantitative arginine requirement of Atlantic salmon smolts. Fish were gradually acclimated to sea water (32 ppt) and maintained for 8 weeks prior to commencement of the study. Triplicate groups of Atlantic salmon were fed to satiation diets (CP: 40% DM) containing 1. 1-3.2 g Arg/ 100 g diet (2.7-8% of protein) for a period of 8 weeks. Growth, feed utilization and nitrogen retention data showed the requirement for arginine of Atlantic salmon to be 1.6% of dry matter (4.1% of dietary protein). An arginine requirement of 1.6% of dry matter (4.1% of dietary protein) was also obtained from broken-line regression of expired (CO2)-C-14 (following an intraperitoneal injection of L-[U-C-14]arginine versus dietary concentration. Except for the loss of appetite resulting in a low feed intake and depressed growth, no nutritional deficiency signs were observed in fish fed an arginine deficient diet for 98 days. The significance of several biochemical indices measured including liver arginase activity and plasma arginine, insulin and growth hormone levels of fish fed graded levels of arginine supplement is also discussed.