Myoplasmin-C1 is a polypeptide detected by a monoclonal antibody, which is localized in the myoplasm of ascidian eggs. Since microinjection of the antibody blocks larval muscle development, myoplasmin-C1 may play a role in muscle cell differentiation (Nishikata et al. 1987). Isolation and characterization of myoplasmin-C1 cDNA clones revealed that the predicted amino acid sequence of myoplasmin-C1 had no similarity to any known protein. However, the deduced protein contains heptad repeats similar to those in myosin heavy chain, tropomyosin and the Drosophila Bicaudal D gene product, suggesting that it is a filamentous component of the myoplasmic cytoskeleton. The predicted amino acid sequence also showed several possible phosphorylation sites. Consistent with the prediction that myoplasmin-C1 is a cytoskeletal component, the protein remained in the myoplasmic cytoskeletal domain after detergent extraction. These results suggest that myoplasmin-C1 is a cytoskeletal component of the myoplasm and that it plays a role in anchoring and segregating muscle determinants.