Within the French phytoplankton monitoring network (REPHY), domoic acid (DA), the toxin responsible for amnesic shellfish poisoning, was first detected in samples collected in 1998. Toxin analysis by the official method [liquid chromatography with diode array detection (LC/DAD)] was performed when Pseudo-nitzschia cell concentration was greater than 1.0 x 10(5) cells/l. LC/DAD results obtained in 1999 and 2000 showed increased DA accumulation in bivalves sampled at different sites along French coasts. The toxin maximum in 1999 was 3.2 microg DA/g of whole tissue, whereas the levels in 2000 (53 microg) were above the sanitary threshold (20 microg DA/g tissue). Phytoplankton samples collected during blooms were observed by both light and scanning electron microscopy (SEM). Identification of phytoplankton species by SEM analyses confirmed the presence of two known DA-producing species, P. pseudodelicatissima and P. multiseries. LC/DAD results for a mass culture of P. multiseries indicated that this species was involved in DA accumulation in French shellfish.