Bioassay-guided fractionation of an aqueous extract of a Philippine Islands collection of the soft coralLobophytum sp. concentrated its HIV-inhibitory activity into fractions rich in cembranoid diterpenes. Lobohedleolide (1), (7Z)-lobohedleolide (2), and a new compound, 17-dimethylaminolobohedleolide (3), were purified from these fractions by HPLC. The structures of compounds 1-3 were elucidated by spectroscopic analyses and by comparison of their spectral data with previously reported values. The relative stereochemistry of the gamma-lactone ring substituents of 3 was determined by 1D NOESY experiments. While several other cembranoids that contain a dimethylamino functional group have been reported from the soft coral Sinularia sp., compound 3 represents the first cembrane diterpene with this functional group isolated from a Lobophytum species. Diterpenoids 1-3 exhibited moderate HIV-inhibitory activity (EC(50) approximately 3-5 microg/mL) in a cell-based in vitro anti-HIV assay.