Mussels (Mytilus edulis) and clams (Ruditdpes philippindrum) were contaminated experimentally using cultures of Alexandrium minuturn, a toxic dinoflagellate isolated from French coastal waters. Experiments were carried out in continually flushed and open-circuit wtems using Alexandrium densities of 100 to 700 cells/ml delivered to tanks containing the shellfish. All experiments indicated an inversion of the relative proportions of gonyautoxins (GTX, and GTXJ in shellfish meat during decontamination, whereas saxitoxin (STX) only accumulated during mussel depuration. However, in mussels a density as low as 100 cells/ml led within 10 days to bioaccumulation of paralytic shellfish poisoning (PSP) toxins above the public health threshold. Similar results were observed in clams subjected to fivefold higher cell densities, indicating a less effective assimilation of the dinoflagellate than by mussel. Decontamination experiments on PSP toxin-contaminated mussels (360 pg STX eqA00 g or lower uptake) fed two nontoxic diets (1,OOO and 10,000 cells/ml of Tetfaselmis suesicd) showed an appreciable reduction in the time needed to decrease toxin concentration below the accepted thrkshold for human consumption. We suggest that a simple relation can be established between initial toxicity, the concentration of nontoxic alga available, and the time required for depuration once decontamination kinetics becomes linear and corresponds to the inverse of contamination kinetics.