Seaweed of the genus Laurenoia, family Rhodomelaceae, have proved to be a rich source of halogenated natural products 2. The present work describes the isolation from 2. caespitosa of a new sesquiterpenoid containing chlorine and bromine, designated as caespitol for which, on the basis of its chemical and spectroscopic behaviour, we propose formula (III) aa the most favorable structure. Ether extracts of the dried seaweed were washed successively with dil KOH soln, dil HCl and water, and the neutral oil thus obtained was carefully chromatographed on standard silica gel. The 75% ether: 25% benzene eluent contained crystalline (m.p. 109-lllS!/n-hexane) caespitol, in 0.03% yield. Caespitol (III), was analyzed for C15H2502Br2C1: m/e M+ 430, 432, 4343 high resolution m/e 353.075 (calcd for C15H250280Br35Cl, 353.070), and m/e 237.192 (calcd for Cl5H2502, 237.185). The ir spectra (V max KBr 3540, 3320, 1460, 1220, 970 and 785 cm-l;Vccl max4 3620, 3400 and 3240 cm"). The pmr spectrum (60 MHz, CDCl3, "I-scale), 5.61 (UI, dd, J=l2 and 4 Hz, -&Br-1, 5.69 (IH, dd, J=12 end 5 Hz, -@Br-), 6.43 (la, t, 5~2.5 Hz, &(OH)-1. The spectrum showed, furthermore, a broad envelope from 7.3 to 8.3 arising from eight methylene protons, and four tertiary methyl signals at 8.33 (M&Cl-), 8.63 [M&(OH)-1, 8.69 and,8.83 (Rem-dimethyl).