Didemnins A, B, and C, cyclic depsipeptides previously reported by our laboratory from the Caribbean tunicate Trididemnum solidum, have now been synthesized and, in the process, their structures have been revised to include the new amino acid (3S,4R,5S)-isostatine,C2H5CH(CH3)CHNH2CHOHCH2COOH. All other isomers of isostatine have also been prepared. Didemnin B is currently in Phase II clinical trials as an anticancer agent, and the antineoplastic activity, as well as the antiviral and immunosuppressive activities, of the didemnins are reviewed. A highly unusual porphynoid has also been isolated from T. solidum. The compound has been named tunichlorin and is apparently the first nickel-containing chlorin (and only the second nickel-containing porphyrin-related compound) identified from a living organism.