The sponge Dysidea herbacea has been the subject of previous studiesly2. A collection of D. herbacea from the Caroline Islands in the Pacific Ocean yielded a series of polybrominated biphenyl ethers exemplified by (l)l whereas material collected on the Australian Great Barrier Reef east of Townsville gave the novel tetramic acid derivative (2)2, the first example of a naturally occurring compound containing a trichloromethyl group. A small sample of D. herbacea collected north-east of Cooktown on the Great Barrier Reef yielded two major fractions. A mixture of penta-and hexabromo-biphenyl ethers, related to the compound (1) previously reported by Sharma, Vig and Burkholder', was accompanied by the hexachloro-metabolite (3), for which we propose the name dysidenin. This was the major metabolite isolated from the dichloromethane extract of the freeze-dried organism. Dysidenin (3) crystallised from hexane as fine colourless needles mp 98-99' [ali -98' (c=O.5, CHCl,). The molecular weight and the formula C,,H,,Cl,N,O,S were established by elemental analysis and by chemical ionization and high resolution mass spectrometry. Reduction of (3) with zinc-acetic acid gave the dechlorinated compound (4), C,,H,,N,O,S (by high resolution m.s.), in good yield. The 13C n.m.r. of (3) showed resonances at 171.9, 171.2 and 168.2 p.p.m. (5, C=O or ;C = N) and two resonances at 142.3 (d,d;J=185.5, 5.9 Hz) and 118.9 p.p.m. (d,d;J=189.4, 15.6 Hz). Two singlets at 105.5 and 105.1 p.p.m. could be assigned to two trichloromethyl groups. High field resonances occurred at 54.0 (d), 51.9 (d), 51.4 (d), 47.3 (d), 37.4 (t), 31.0 (t), 30.8 (q), 21.8 (q), 17.3 (q) and 16.2 (q) which accounted for all 17 carbon atoms of the molecule. Valuable information was obtained from the 100 MHz 'H n.m.r. spectra of (3) and (4) which allowed the definition of several structural units of the molecules. Table 1 shows the resonances, coupling constants and assignments obtained in three solvent systems for (3) and in Ccl, for (4). Extensive decoupling studies and chemical shift values suggested the partial structural units (5), (6) and (7). The presence of a 2-substituted thiazole ring system in (3) was supported by the U.V. spectrum (xmax(Me@H)240 nm, log E 3.6), the positions and coupling constants of the low