The amino acid composition of myosin from the white muscle of trout has been determined and was found to be closely similar to that of rabbit myosin. Thirteen of the amino acid residues were present in concentrations identical to those of rabbit myosin or agreed within less than 3%. Valine, glycine, and methionine were present in slightly greater proportions. The histidine and proline contents of trout myosin differed from rabbit myosin by three residues per 105 g of protein and represented the greatest difference on a per cent basis. The possible significance of these differences is discussed in relation to the lower enzymatic activity and the lower thermal stability of trout myosin. The apparent specific volume of trout myosin calculated from the amino acid composition [Formula: see text] was identical to that obtained by density measurements. An apparent molecular weight calculated from the amino acid analyses was 500,027 ± 44.