The light chain fraction was separated from myosin extracted from chicken cardiac muscle. Two light chain components, L-1 and L-2 in the fraction were isolated by chromatography on a column of DEAE-cellulose (DE-52) in the presence of4 M urea. After performic acid oxidation, the L-1 chain was digested with trypsin and the resulting peptides were isolated. The amino acid sequences of the peptides were established. The ordering of these tryptic peptides in the L-1 chain was deduced from the amino acid compositions and the partial sequences of peptic peptides from S-carboxymethylated L-1 chain. Comparing the whole sequence of the L-1 chain thus established with that of alkali light chain of rabbit skeletal muscle myosin, 67 amino acid substitutions and two insertions were recognized.