Thyroid-stimulating hormone (TSH) beta subunit cDNA was obtained from Japanese quail (Coturnix japonica) by PCR and its nucleotide sequence was determined. The cDNA encodes a putative signal peptide and a mature protein consisting of 20 and 114 amino acids, respectively. The amino acid sequence of quail TSHbeta subunit shows homologies of 67-69% in mammalian species, 58% in amphibian and 43-49% in teleost fish. Comparison of the amino acid sequence with TSHbeta subunits of other species reveals some differences in several regions responsible for its biological functions and characteristic features of the avian TSHbeta subunit, suggesting that the functional domains have diverged cooperatively between the hormone and its receptor during evolution.