The structure of dendrodoine, a new cytotoxic indole derivative isolated from the marine tunicate Dendrodoa grossularia (Styelidae), has been established as 5-[(3-N-dimethylamino)-1,2,4-thiadiazolyl]-3-indanyl methanone by X-ray analysis and spectral data (NMR, mass, IR). The animals collected in September from Northern Brittany were lyophilized (2.200 kg dry weight) and extracted with ether. Two successive silica gel column chromatographies (hexane-AcOEt 70/30, then 50/50) yielded dendrodoine (mp 280-285°C, 98 mg). Elemental analysis and exact mass (M+272.073) correspond to the molecular formula C13H12N4OS; its monoacetyl derivative (Ac2O, Py cold) has mp 192-197°C (M+314). NMR spectra indicated a 3-indanone moiety, confirmed by mass spectrum (base peak m/e 144) and IR (ν=3225, 1630 cm⁻¹). X-ray diffraction of dendrodoine acetate (lb) revealed triclinic crystals (space group P1, a=11.417, b=8.663, c=7.666 Å, α=94.10°, β=104.12°, γ=93.21°, Z=2). Structure solved via Multan and refined to R=0.056 (1943 reflections). The molecule is nearly planar (indole-thiadiazole dihedral angle 11°), with crystal cohesion via van der Waals forces. This is the first natural thiadiazole-containing derivative.