The differences of toxin accumulation and elimination among oyster, mussel and scalllop were investigated in relation to the density of Alexandrium tamarense. The toxicity of oyster, mussel and scallop increased in association with the cell numbers. However, the ratios of the toxicity level to the cell number were different among the bivalve species. The highest accumulations were found in scallop and mussel (about 3 times higher than in oyster). The elimination rate of toxicity in the oyster was fast. But, after the disappearance of A. tamarense, the toxicity of scallop was retained for a long period. During the period of increasing plankton growth, the proportions of toxins in the body of bivalves reflected those in the plankton. But when the plankton density started decreasing, the toxin ratios in the bivalves changed on a daily basis, presumably owing to metabolism of the toxins in the bivalves.