Amino acid sequences of the regulatory light chains of striated adductor muscle myosin from Ezo giant scallop (Patinopecten yessoensis) and akazara scallop (Chlamys nipponensis akazara) were determined. Tryptic peptides of each light chain were isolated and sequenced. The alignment of the tryptic peptides in each chain was deduced from the amino acid compositions and the partial sequences of peptic peptides of the Ezo giant scallop light chain. The light chains both consist of 156 residues. Heterogeneous residues, glutamic acid and aspartic acid were observed at the 155th position in the sequence of the akazara scallop light chain. A comparison of the Ezo giant scallop light chain with the glutamic acid-containing and aspartic acid-containing akazara light chains revealed 6 and 7 amino acid substitutions, respectively. When the presented sequences were compared with those of the regulatory light chains of gizzard, cardiac and skeletal muscle myosins, a strong homology was observed in the calcium binding region, but there were considerable heterogeneities in the N- and C-terminal regions.