The study of the phylogenetic distribution of the beta-thymosin family is important to elucidate its biological function further. A new thymosin, designated as thymosin beta 14, consisting of 40 amino acid residues and with a molecular weight of 4537 Da as determined by ion spray mass spectrometry, was isolated from the sea urchin. The N-terminus of this polypeptide is blocked by an acetyl group as found by matrix-assisted laser desorption mass spectrometric and amino acid analysis. The primary structure was elucidate by Edman degradation of the HPLC-purified thymosin beta 14 fragments produced by digestion with endoproteinase Asp-N and trypsin. Sequence comparison reveals that thymosin beta 14 is 73% homologous to thymosin beta 4, obtained from calf thymus. By isolating and characterising the structure of thymosin beta 14 from the sea urchin, an invertebrate, substantial knowledge about the phylogenetic distribution and evolution of beta-thymosins is gained.