Studies of the Brazilian gorgonian octocoral Heterogorgia uatumani have resulted in the discovery of two metabolites that inhibit fish feeding under natural conditions. These are the previously reported eunicellane diterpenoid, (6E)-2alpha,9alpha-epoxyeunicella-6, 11(12)-dien-3beta-ol (1) and a new sesquiterpene lactone, heterogorgiolide (2). The structures of 1 and 2 were determined by spectroscopic methods and by comparison of spectral data with literature values. Field bioassays of the two compounds, at their natural concentrations, confirmed that they deter predation by a complex assemblage of reef fishes. This is an unusual observation showing that defenses are derived from both sesqui- and diterpenoid metabolites.