We have cloned a full-length cDNA coding for activin betaA subunit from a goldfish brain and pituitary cDNA library, which represents the first for activin betaA in fish. Sequence analysis of goldfish activin betaA shows that this peptide is highly conserved across vertebrates. The mature region of goldfish activin betaA shares 81% amino acid identity with that of humans. Messenger RNA of goldfish activin betaA is expressed in a variety of tissues including ovary, testis, brain and liver, suggesting a wide range of physiological roles for activin A in the goldfish. The identity of the cloned goldfish activin betaA was confirmed by expressing the protein in the Chinese hamster ovary (CHO) cells followed by detection of the specific activin activity in the culture medium using erythroid differentiation factor (EDF) assay with F5-5 cells. Stable CHO cell lines producing high level of recombinant goldfish activin A were established and characterized.