Dolastatin D (1), a cytotoxic cyclic depsipeptide possessing the novef &amino acid (~,3R)-3-amino-2 methylbutanoic acid [(2R,3R)-31 as a component, has been isolated from the Japanese sea hare Dolabellu ouricularia. The absolute stereostructure of 1 was elucidated on the basis of spectroscopic analysis and chemical degradation and was fur&r coned by synthesis.