Plicatamide is a modified octapeptide from the ascidian Styela plicata having the structure Phe-Phe-His-Leu-His-Phe-His-decarboxy Delta DOPA (where decarboxy Delta DOPA = decarboxy-(E)-alpha,beta-dehydro-3,4-dihydroxyphenylalanine), Edman sequencing, tandem mass spectrometry, and proton NMR have been used to characterize 100 mu g of the compound. Plicatamide represents an important link between two classes of biomolecules: the tunichromes which share an oxidatively decarboxylated C-terminus and higher molecular weight DOPA-polypeptides. The 8-residue sequence provides the first opportunity to investigate the biosynthetic origins of the tunichrome family by molecular biological techniques.