A complete cDNA was cloned from the bay scallop (Argopecten irradians) that codes for a 382-amino-acid myostatin-like protein (sMSTN). The sMSTN sequence is most similar to mammalian myostatin (MSTN), containing a conserved proteolytic cleavage site (RXXR) and conserved cysteine residues in the C-terminus. Based on quantitative RT-PCR, the sMSTN gene is predominantly expressed in the adductor muscle, with limited expression in other tissues. Using the sMSTN sequence, a Ciona MSTN-like gene was also identified from the Ciona intestinalis genome. These findings indicate that the MSTN gene has been conserved throughout evolution and suggests that MSTN could play a major role in muscle growth and development in invertebrates, as it does in mammals.