The sponge Dysidea herbacea has been the subject of previous studies. A collection from the Caroline Islands yielded a series of polybrominated biphenyl ethers, while material from the Australian Great Barrier Reef east of Townsville gave a novel tetramic acid derivative (the first naturally occurring compound with a trichloromethyl group). A small sample collected north-east of Cooktown on the Great Barrier Reef yielded two major fractions: a mixture of penta- and hexabromo-biphenyl ethers related to a previously reported compound, and the hexachloro-metabolite dysidenin (3) (the major metabolite from the dichloromethane extract of freeze-dried organism). Dysidenin (3) crystallised from hexane as fine colourless needles (mp 98-99°C, [α] -98° (c=0.5, CHCl₃)). Its molecular weight and formula C₁₇H₁₉Cl₆N₃O₃S were established by elemental analysis, chemical ionization and high resolution mass spectrometry. Reduction of (3) with zinc-acetic acid gave the dechlorinated compound (4) (C₁₇H₁₉N₃O₃S, by high resolution MS) in good yield. The ¹³C NMR of (3) showed resonances for C=O or ;C=N groups, two CH groups, two trichloromethyl groups, and other carbons. Valuable information from 100 MHz ¹H NMR spectra of (3) and (4) allowed definition of several structural units. The presence of a 2-substituted thiazole ring in (3) was supported by the UV spectrum (λmax (MeOH) 240 nm, log ε 3.6). The structure of (3), assigned by combination of evidence from ¹³C and ¹H NMR, UV and IR (νmax (KBr) 3270, 1680, 1620, 1540) spectra, was substantiated by high resolution mass spectra of (3), (4) and (8) (prepared by reduction of (3) with zinc-CH₃COOD). Fragment ions of all three compounds were fully consistent with those expected for N-acylated α-amino acids with an amino-alkyl-thiazole residue attached at the carboxyl group, establishing the structure of (3).