Tentative qualitative and quantitative amino acid requirements have been reported for the major species of fish reared for market or as replacement stocks for natural waters. Most work has concentrated upon juvenile fish or upon rapidly growing young market fish; these have high protein dietary requirements (30-50%) that are in direct contrast to the homothermic terrestial animals. Net protein utilization from the diet is similar or slightly better than that found in avian species, but energy needs are much lower in fish and as a result the body protein deposition in fish is larger (about 5 g protein/MJ for the chick versus about 10 g protein/MJ for young fish). Qualitative amino acid requirements appear identical for all fish species examined; arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine are all required for normal growth and metabolism. Quantitative requirements differ only slightly among species that have been tested. Salmon have higher arginine requirements than other fish examined. Catfish appear to have a lower requirement for histidine and threonine, and the Japanese eel seems to need more tryptophan in the diet. However, when the quantitative requirements for indispensable amino acids are expressed as a percentage of the protein fed, then a remarkable harmony appears between values needed for maximal growth for most species examined. A review of Tables 3 and 4 will disclose the paucity of information available considering the large number of fish species reared commercially over the world. Most commercial diet formulations have relied upon the work done on salmon, catfish, and carp, and their amino acid and protein requirement values have been used. Remarkably, these diets have produced other species of fish economically. Sparing effects of one amino acid on another have only been studied with cystine-methionine and tyrosine-phenylalanine. Arginine and analogues of methionine have been used as good nitrogen sources for salmon. Isoleucine-leucine ratios have been measured and experiments indicate some growth inhibition when the isoleucine-leucine ratio was greater than 2/1. Valine at abnormally high levels also inhibited growth. Much more work needs to be done on the effects of subtle differences in amino acid ratios in the diet, and major emphasis should be placed on the important role of the dispensable amino acids in fish nutrition.(ABSTRACT TRUNCATED AT 400 WORDS)