The L-1 light chain of myosin, extracted from chicken fast skeletal-muscle, was digested with trypsin after S-carboxymethylation. The tryptic peptides in the digest were separated and purified and the amino-acid compositions and sequences of the tryptic peptides analysed. The carboxymethylated L-1 light chain was also digested with pepsin and the peptic peptides in the digest were separated and purified. The amino-acid compositions and the partial sequences of peptic peptides were analysed. From these results, the primary structure of the L-1 light chain of chicken fast skeletal-muscle myosin was determined. The established sequence consists of 190 amino acids.