A new analogue of okadaic acid (OA), the toxin mainly responsible for diarrhetic shellfish-poisoning (DSP) phenomena in Europe, has been isolated from toxic phytoplankton (Dinophysis acuta) collected in Irish waters. Fluorimetric LC analyses of the extracts of bulk phytoplankton samples using derivatisation with 9-anthryldiazomethane (ADAM) showed a complex toxin profile, with peaks corresponding to OA and dinophysistoxin-2 (DTX-2) as well as a third unidentified compound. This minor unidentified component was isolated by chromatographic techniques such as normal-phase chromatography, gel permeation on Sephadex, solid-phase extraction and reversed-phase separations. Ionspray mass spectrometry (MS) was used for structural investigation on this compound due to the very small amount of isolated material. Flow injection analysis (FIA)-MS of the isolated compound gave positive-ion mass spectrum dominated by the protonated molecule, [M + H]+, at signal m/z 805, whereas the deprotonated molecule [M - H]- was observed in the negative-ion spectrum at signal m/z 803, thus indicating the molecular weight of 804 for the new toxin, the same as OA and its known isomers, DTX-2 and DTX-2B. Collision-induced dissociation (CID) as obtained by positive and negative tandem mass spectrometry (MS-MS) showed a fragmentation pattern for the new compound which was very similar to that of OA, DTX-2 and DTX-2B. Ionspray microLC-MS of a mixture containing the compound under investigation together with OA analogues showed the compound eluted after OA, DTX-2, DTX-2B and before DTX-1. All the chromatographic and mass spectrometric data indicated the compound to be another OA isomer and it was therefore coded DTX-2C. To the best of our knowledge this is the first report on the isolation of a new compound related to DSP toxins from natural communities of toxic phytoplankton.