In the goby, Gillichthys mirabilis, urotensin II (a bioactive neuropeptide present in the urophysis of teleost fish) has the dodecapeptide sequence, H2N-AGTADC-FWKYCV-OH, which is homologous with mammalian somatostatin at positions 1, 2 and 7-9. The Merrifield solid phase synthesis of Gillichthys urotensin II (UII) was accomplished by stepwise assembly from the carboxy terminus using N-alpha-tert, butyloxycarbonyl (Boc) amino acids containing benzyl-derived groups for protection of side-chain functionalities, Coupling of amino acids to the growing peptide was mediated by diisopropylcarbodiimide (DIC) in the presence of 1-hydroxybenzotriazole (HOBt). Residual alpha-amino groups remaining after coupling were blocked by acetylation with 1-acetylimidazole. Crude, synthetic UII was extracted from the HF-treated, protected peptide-resin product, reduced with dithiothreitol (DTT), reoxidized at high dilution with O2, and separated into its components using a single, preparative, reverse-phase HPLC step. The pure, synthetic UII, obtained in 7.6% yield from oxidized crude UII, was indistinguishable from pure, native UII in specific bioactivity, amino acid sequence, and retention time in each of two different HPLC systems.