To gain insight into the evolution of the structure and functions of transforming growth factor (TGF)-beta superfamily members, a cDNA encoding a new member from the bivalve mollusc Crassostrea gigas named mGDF (molluscan growth and differentiation factor) was identified by PCR using degenerate primers. The mGDF precursor exhibits characteristic features of the TGF-beta superfamily and shows highest homology with human BMP2 and Drosophila DPP. Conversely, the mgdf gene displays a distinct pattern of expression during development. Indeed mgdf transcripts were not detected early in development but increased markedly before metamorphosis. These findings raise the possibility that mGDF could play a central role in the biological processes that allow larvae to become competent to metamorphose.