Fish flesh and certain waste materials were hydrolysed by tryptic enzymes of fish pyloric caeca. Fractionation of the resulting hydrolysates showed that they contained largely peptone, sub-peptone and residual (small peptides and amino acids) nitrogen, and little or no protein or proteose nitrogen. Fish flesh, milts, roes, meal, stickwater and a muscle myosin preparation were extracted to remove the fat, then dried and hydrolysed with acid or alkali. The essential amino acids arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, valine, tryptophane and tyrosine were determined in these enzyme-, acid- and alkali-hydrolysed materials by microbiological methods. The results have been summarized.