Cyclodldemnanude (I), a new cyclic heptapeptide. has heen Isolated from the marine asctdian Didemnum molly collected jn the Philippine Islands. The structure of the new, weakly cytotoxtc, heptapeptide was unambiguously asslgned using comprehensive NMR analysis in combination with hydrolysis and chiral W-MS analyw of its component amino acids. Computer analysis of 1 led to assignment of a solution conformation that fully rationalized the nOe correlations and shielding effects observed by 'H NMR.