Almost yearly since 1988, summer blooms of toxic Alexandrium minutum have occurred along Brittany (N.E. Atlantic). For the first time, during the autumn of 1998, monitoring toxicity tests performed on shellfish collected in the Thau lagoon (W. Mediterranean coast) gave neurotoxicity symptoms on mice. Plankton samples collected, which were almost monospecific, indicated that a dinoflagellate, subsequently identified as A. tamarense, was the likely cause. Throughout the toxic event, average cell concentration seldom exceeded 50 000 cells.L-1, despite a peak of 3.5 105 cells.L-1 on November 9th. HPLC analysis showed that plankton and shellfish extracts had a similar toxic profile, which was much more complex (C1,C2, GTX 1/4, 2/3, B1, B2, STX, dc-STX) than that characteristic of A. minutum in France (C1, C2, GTX 2/3). Shellfish marketing was banned for almost two months. Shellfish toxicity shows different pattern for the 3 species that were regularly sampled. Using the PSP AOAC mouse-test, a maximum toxicity of 850 µg STX eq./100 g of tissue was encountered in mussels which remain contaminated during all the event, whereas oysters never went above the sanitary threshold (80 µg STX eq./100g), and clams went slightly above. The spatiotemporal distribution of the event throughout the lagoon in these shellfish species monitored is considered in terms of mouse-tests and HPLC results. The latter indicate that Alexandrium tamarense is the causative agent.