Sea hares have yielded a fascinating array of natural products, most of which appear to be accumulated from diverse dietary sources. Interest in the chemistry of these opisthobranch molluscs has been stimulated by the reputed toxicity of some species, the observation of cytotoxic activity in sea hare extracts, and the promise of encountering novel products. Prompted by our previous observation that the sea hare Aplysia dactylomela contained a wide variety of halogenated and nonhalogenated metabolites, we investigated another species, Bursatella leachii pleii (Rang). In contrast to the case of A. dactylomela, we have found that extracts of the latter mollusc collected near La Paguera, Puerto Rico, are comprised mostly of aliphatic lipids, but among the minor components we have isolated a novel dinitrile whose structure we report herein.