Four new cytotoxic, cyclic peptides, patellamide D (10) and lissoclinamides 4,5, and 6 (11-13), have been isolated from the ascidian Lissoclinum patella collected in Australia and their structures have been determined by 1H and 13C NMR analyses. Absolute configurations were determined by hydrolysis and analysis of the derivatized constituent amino acids on chiral GC and HPLC columns. Definitive 13C NMR assignments have been made. The structure and absolute configuration of patellamide D were also determined by X-ray crystallography. The solid-state conformation of patellamide D was found to be different from that of the closely related peptide ascidiacyclamide. Computer modeling indicates that the energy-minimized conformation of patellamide D closely resembles that found in the crystalline form and this is -10 kcal/mol lower in energy than that of an energy minimized "model" of patellamide D based on the reported crystal structure of ascidiacyclamide.