Azaspiracid is a previously unknown and structurally novel phycotoxin found to be responsible for an outbreak of diarrhetic food poisoning associated with consumption of contaminated Irish shellfish in Europe in 1996. While azaspiracid was previously classified as a diarrhetic shellfish poison (DSP), it has recently been reclassified into a new poisoning category known as azaspiracid poisoning (AZP), because it has a number of unique properties that set it apart from the \'classic\' DSPs, i.e. okadaic acid (OA), yessotoxin and the dinophysistoxins (DTXs). The response of mice to azaspiracid is characterised by hopping, scratching and progressive paralysis of the animal, leading to a quick death normally within 5-60 minutes, while mice injected with OA/DTX contaminated mussel extract die over a longer period of time following convulsions and prostration of the animal. This study describes the distribution of azaspiracid within the blue mussel. OA/DTX are known to be located exclusively within the hepatopancreas (HP) of the mussel, even to the extent that contaminated mussels with their HP removed are toxin-free and safe for consumption. Batches of mussels contaminated with azaspiracid, OA/DTX or both and greenshell mussels containing yessotoxin were dissected and their HP removed and pooled. The remaining flesh from each batch was also pooled and all fractions were extracted according to the Yasumoto protocol. The extracted solutions were then analysed via the mouse bioassay and by a novel cytotoxicity assay developed and optimised for the detection and discrimination of DSPs and azaspiracid. Only the HP fraction of the yessotoxin and OA/DTX contaminated mussels were toxic in the mouse bioassay, the flesh minus the HP had no adverse effect on the mice. A similar result was obtained with the cell bioassay. When azaspiracid positive samples were analysed a strongly positive result in both the mouse bioassay and the cytotoxicity assay was seen for both the HP and flesh minus HP fractions. When mussels containing OA/DTX and azaspiracid were dissected and analysed OA/DTX was only seen in the HP fraction while azaspiracid was seen in both dissected portions. This result indicates that the azaspiracid is distributed throughout the mussel and supports the observation that azaspiracid contaminated mussels take longer to depuriate than OA/DTX or yessotoxin contaminated mussels.