A new octapeptide has been isolated and characterized as its potassium complex from acidic aqueous methanol solutions of ascidiacyclamide, a cyclic peptide isolated from the ascidian Lissoclinum patella. Crystals suitable for X-ray structure determination were orthorhombic, space group P2(1)2(1)2(1), with a = 15.442(3) Angstrom, b = 36.167(1) Angstrom, c = 9.567(3) Angstrom, V = 5343(2) Angstrom(3), Z = 4, and R = 0.069. The structure consists of the macrocyclic cation, three perchlorate anions, and two water molecules. The macrocyclic ring of ascidiacyclamide remains intact, but two oxazoline rings have been opened to form two amino lactones, with the amine protonated. A potassium ion is bound to the N atoms of the thiazole rings (K(1)... N(1), 2.47(2) Angstrom; K(1)... N(5), 2.50(2) Angstrom) and to the adjacent O (amide) atoms (K(1)... O(1), 2.34(1) Angstrom; K(1)... O(5), 2.34(1) Angstrom). Two perchlorate anions, located on either side of the plane of the macrocyclic ring, are weakly associated with the potassium cation (K(1)... O(10), 2.72(3) Angstrom; K(1)... O(16), 2.48(3) Angstrom). This cyclic octapeptide reacts with copper(III) to give a purple solution (lambda(max) 590 nm).