Essential amino acids for the 2 species of eel, A. anguilla and A. japonica, were examined by using an amino acid test diet. Forty fish were grouped in each aquarium and fed the test diets for a period of 6 weeks. In the both species, the fish fed diets deficient in each of alanine, aspartic acid, cystine, glutamic acid, glycine, proline, and tyrosine grew as well as those fed the complete amino acid diet. The fish fed diets deficient in each of arginine, histidine, isoleucine, leucine, lysine. methionine, phenylalanine, threonine, tryptophan, and valine failed to grow until the deleted amino acid was added to the ration. It was thus established that eels require the same 10 kinds of amino acid reported to be essential for salmonids and others.