The Indian Ocean shell-less mollusc Dolabella auricularia superficially appears to lack predator defenses. Such seemingly defenseless animals are only attacked by certain carnivorous²ᵃ members of this gastropod subclass. Evidence is now accumulating that Opisthobranchia species have developed very powerful chemical defenses by careful selection and/or biosynthetic manipulation of various dietary sources such as blue-green algae¹ᵃ and sponges³ (employed by nudibranchs, including possible protection of egg masses⁴). Our early (1968-1972)⁵ assumptions that certain species of shell-less molluscs of, e.g., the Aplysia⁵ and Dolabella⁶ genera contain potentially useful defensive constituents of the cell growth inhibitory type has been amply realized by isolation of the exceptionally potent antimelanoma pentapeptide dolastatin 10⁶ from D. auricularia⁶. We now report the isolation and structural elucidation of a new cell growth inhibitory (P388 lymphocytic leukemia, PS system⁷) constituent of this animal that represents a hitherto unknown type of cyclodepsipeptide.