The widely distributed red seaweed hmencia is known as a unique source of halogenated Cl5 nonterpenoid ethers and halogenated and nonhalogenated sesqui-and diterpenoid compounds'. More recently this source has also been shown to produce a interesting bromine-containing squalene derivative2. The diterpenoids from kurencia consist of aplysin-203'4, concinndio15 and neoconcinndiol hydroperoxide6, which are structures closely related to the rather commonly observed labdane diterpenoids7, as well as the irieols8, dibromoditerpenoids of unique composition. In this communication we wish to report the isolation and structure elucidation of an unprecedented new dibromoditerpenoid, obtusadiol (A). Obtusadiol was isolated, along with a-snyderol acetate, brasilenol', epibrasilenol' and guaiazulene from the Mediterranean alga L. obtusa (Huds.) Lamouroux". The CHC13/MeOH (l/l) extract of air-dried L. obtusa (collected Rafina, Greece, June 1975) was fractionated by conventional silica gel column chromatography (Grace 62, Et20 elution) to obtain obtusadiol (l_), an oil, [a]i5-24.5"(c 3.1, CHC13), in 0.1% yield of the dry algae. High resolution mass spectrometry established the elemental composition C20H30Br20 (observed 444.0664, talc. 444.0665) for the at 3500, 1650 and 1370, dimethyl substituents. M+-H20 fragment. The thin-film infrared spectrum contained absorptions 1390 cm-', which were assigned to hydroxyl, terminal olefin and gem-1 H and l3 C NMR data were of great utility in determining the structure 2453