Résumé : Flexibilene is a novel monocyclic diterpene hydrocarbon isolated from the soft coral Sinularia flexibilis. Spectroscopic analyses established its structure as 1,1',5,9,13‐pentamethyl‐Δ2,5,9,13‐cyclopentadecatetraene (II). Flexibilene is the first 15‐membered ring diterpene to be found in nature. Copyright © 1976 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim