<jats:p>Six new polyaromatic alkaloids, lamellarin I (3), J (6), K (7), L (9), M (11) and the triacetate (13) of lamellarin N, and four known alkaloids, lamellarin A (I), B (15), C (16) and the triacetate (14) of lamellarin D, have been isolated from the marine ascidian Didemnum sp. The structures were deduced by high-field n.m.r. spectroscopy including 13C-1H shift-correlated two-dimensional (2D) n.m.r. experiments and n.O.e . measurements. The triacetates (8) and (10) of two of the compounds, lamellarin K (7) and lamellarin L (9), were dehydrogenated in high yield by treatment with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone in ethanol heated under reflux. The products obtained were identical in all respects to the triacetates of lamellarin M and N, respectively. The re-isolation of lamellarins A-D, which were previously obtained from the prosobranch mollusc Lamellaria sp., lends further support for the idea that these molluscs sequester the compounds from ascidians as food sources.