Toxicity and toxin profiles of the mussel, Mytilus edulis and the toxic dinoflagellate, Alexandrium tamarense in Masan (Stns M) and Chilchundo (Stns C) of Chinhae Bay, Korea were investigated weekly from March to May in 1996 (Stn M1 and C1) and 1997 (Stn M2 and C2). Alexandrium tamarense was more abundant at Stn M1 in 1996 and at Stn C2 in 1997, while density of total phytoplankton was always higher at Masan. Population of A. tamarense always occupied larger portion among total phytoplankton at Chilchundo than at Masan. In 1996, the peak of toxicity in the mussels was recorded when A. tamarense was abundant in both stations. Maximum toxicity of M. edulis was largely different between the stations, much higher at Stn C1 (560 MU g-1) than that at Stn M1 (22 MU g-1). Toxicity in cultured strains of A. tamarense at Stn C1 was higher in general than that at Stn M1, ranging from 1.0 to 109.8 MU 10-6 cells-1 and from 3.8 to 59.8 MU 10-6 cells-1 at Stn C1 and Stn M1, respectively. Toxin profiles of the strains showed that C1-C2 and GTX1+4 were the major components, while GTX2+3, GTX5, neoSTX and STX were the minor. In 1997, the mussel toxicity was also much higher at Stn C2 than that at Stn M2. Maximum toxicity in M. edulis and peak abundance of A. tamarense at two stations were correlated, showing the time lag of 1-2 weeks. Toxicity of the size-fractionated phytoplankton (10-100mm) ranged from 0.002 to 0.073 (MU^-1) and from 0.004 to 0.029 (MU^-1) at Stn C2 and Stn M2, respectively. PSP causative organism found in size-fractionated phytoplankton was only A. tamarense. Size-fractionated phytoplankton showed toxin profiles of C1-C2 only at both stations. However, PSP toxins in M. edulis consisted of 10 components including C1-C2, GTX1-5, dcSTX, STX and neoSTX. The results showed that toxicity of M. edulis was conspicuously originated from A. tamarense blooming in Chinhae Bay, while biotransformation may cause the relative increase in carbamate toxins in the mussels. Although feeding kinetics of the mussels was beyond the subject of this study, higher portion of A. tamarense population and lower density of total phytoplankton both may explain that higher ingestion rate of the mussels on A. tamarense at Chilchundo led to the higher toxicity.