Chicken TSH beta subunit cDNA was cloned and sequenced. The cDNA encodes a putative signal peptide and a mature protein consisting of 20 and 114 amino acids, respectively. The amino acid sequence of chicken TSH beta subunit is highly conserved (98.5%) between chicken and Japanese quail, whereas it has a low homology between chicken and mammals (67-69%), an amphibian (58%) and fish (40-49%). Structural characteristics of TSH beta subunits of avian species are discussed in comparison with those of non-avian vertebrates.