Through differential screening between oyster families selected for high and low summer survival, we have characterized a new transforming growth factor-beta (TGF-beta) superfamily member. This novel factor, named oyster-gonadal-TGFbeta-like (og-TGFbeta-like), is synthesized as a 307 amino acid precursor and displays 6 of the 7 characteristic cysteine residues of the C-terminal, mature peptide. Sequence comparison revealed that og-TGFbeta-like has a low percentage of identity with other known TGF-beta superfamily members, suggesting that og-TGFbeta-like is a derived member of this large superfamily. Real-time PCR (RT-PCR) analysis in different oyster tissues showed that og-TGFbeta-like is specifically expressed in both male and female gonads, at distinct levels according to the reproductive stage. Og-TGFbeta-like relative expression was the lowest at the initiation of the reproductive cycle and increased as maturation proceeded to achieve a maximal level in fully mature female and male oysters. In situ hybridisation demonstrated that expression was exclusively detected in the somatic cells surrounding oocytes and spermatocytes. The role of this newly-characterized TGFbeta member in the reproduction of cupped oyster is discussed in regard to the specificity and the localization of its expression, which singularly contrasts with the pleiotropic roles in a variety of physiological processes commonly ascribed to most TGF-beta family members identified so far.