A liquid chromatography/mass spectrometry (LC/MS) method was developed for the sensitive and specific determination of azaspiracid and its two analogs, the causative toxins of azaspiracid poisoning that occurred in the Netherlands and Ireland. The LC/MS method provided a detection limit of 50 pg for azaspiracid. The sensitivity was approximately 8 × 104 times greater than the mouse bioassay. The method was used to confirm the presence of azaspiracids in toxic mussels collected at Arranmore Island, Ireland in 1997. Copyright © 1999 John Wiley & Sons, Ltd.