The highly productive mussel fishery in the Rias Bajas region of northwest Spain has experienced several outbreaks of paralytic shellfish poisoning (PSP) beginning in 1976. In this study, similarities in the HPLC analyses of extracts from toxic shellfish, plankton tows and cultured dinoflagellates from the Rias Vigo and Pontevedra clearly indicate that Gymnodinium catenatum Graham is the organism responsible for recent PSP episodes. The toxin profile of the dinoflagellate contains an unusually high proportion of the low potency sulfocarbamoyl toxins (ca. 90-95 mole %), although a major portion of the overall toxicity is due to the more potent saxitoxin that is present at 5-10% of the total. Toxin profiles of shellfish showed approximately the same composition as that of the dinoflagellate, although the shellfish contained several carbamate toxins (GTX I, GTX II, GTX IV and NEO) that were not detected in G. catenatum culture extracts. The shellfish also contained decarbamoyl toxins (dc-GTX II and dc-GTX-III) at approximately 2% of the total profile. Since these were not detected in the dinoflagellate, their presence reflects either chemical or enzymatic conversion within the shellfish.