From newt testes we cloned by reverse transcription-polymerase chain reaction (RT-PCR) activin beta A and beta B subunit genes, each encoding the mature region of the peptides. A single from for each of the subunit, genes was isolated. Newt beta A and beta B share 80 and 98% protein sequence similarity with the human subunits, respectively. Northern blot analysis showed that beta A subunit mRNA is approximately 7 kilobases in length and is expressed during spermatogenesis in stages from spermatogonial to spermatid, but activin beta B subunit mRNA is barely detectable during these sages. We also isolated a cDNA clone containing the entire coding region of beta A and performed in situ hybridization with its cRNA probe. Newt activin beta A subunit mRNA was detected in Sertoli cells, but not in germ cells or pericystic cells. These findings suggest that activin A plays an important role in newt spermatogenesis.