Amino acid sequence analysis of the regulatory light chain of obliquely striated muscle myosin from earthworm, Lumbricus terrestris, was performed completely. The polypeptide consists of 195 amino acids with a calculated molecular mass of 21943 Da. From the arrangement of amino acid residues, the first EF-hand domain appears to be a specific Ca(2+)-binding site. The unusually long N-terminal region of about 40 amino acids, which is characterized by accumulation of basic amino acids, is similar to that of the myosin A1 catalytic light chain from rabbit skeletal muscle.