Follistatin (FST) is an activin-binding protein that neutralizes the activity of activin. FST also binds other members of the transforming growth factor-beta (TGF-beta) superfamily, including myostatin (MSTN). We report herein on the isolation and characterization of a full-length cDNA sequence predicted to encode FST in a marine fish, the gilthead sea bream Sparus aurata. The deduced amino acid sequence of sea bream FST (saFST) is highly conserved to the counterpart sequences in other vertebrates and contains the N-terminal domain and three FST domains. The deduced mature saFST shows 81-86% identity with FSTs from other vertebrates. It is 290 amino acids long, similar to other fish FSTs and the short isoform of Xenopus FST but longer by two residues than mammalian FST288. Ontogeny of MSTN (a TGF-beta superfamily member and a negative growth regulator of skeletal muscle in mammals), and FST (known to bind MSTN) gene expression revealed the presence of both transcripts throughout larval development. However, a different expression pattern was found in earlier developmental stages; while MSTN could not be detected prior to the day of hatching, FST transcript was detected in embryos 12 h post-fertilization, confirming its role during vertebrate embryonic development. Both FST and MSTN were expressed in many adult tissues, with variable levels of expression, including muscle. Recombinant saFST inhibited saMSTN activity in a reporter gene assay, indicating a similar effect to that reported in mammals.