A peptide containing 41 amino acid residues has been isolated from trout liver and identified as a member of the beta-thymosin family. Sequence analysis shows it to be 78% homologous to thymosin beta 4, which is the peptide present in the thymus and other tissues of higher vertebrates, including, as reported here, livers of a species of reptile. Thymosin beta 11 appears to replace the more prevalent thymosin beta 4 in at least two species of bony fish, and represents the sixth structurally characterized member of this widely distributed family of peptides.