The toxicity and toxin composition between blue mussel, Mytilus edulis and oyster, Crassostrea gigas collected at Woepori in Ko je island in South Coast of Korea in 1996 and 1997 were compared. The highest toxicity score was about 10 times higher in blue mussel than oyster (blue mussel, 8,670 $mu extrm{g}$; oyster, 860$mu extrm{g}$ in 1996, blue mussel, 5,657 $mu extrm{g}$/100g in 1997). The blue mussel also retained its toxicity for slightly longer period than oyster. In the both shellfish, PSP was composed almost exclusively of C toxicity (Cl and C2, 20~65%) and gonyautoxins (GTXl, 2, 3, and 4, 38~78%). In the early period of toxin accumulation, the ratio of 11$eta$-epimer toxins (C2, GTX4) whose amount was 25~56 mole% (5th March to 12th April in 1996) and 25~80 mole% (18th March to 7th April in 1997), were higher than that of 11-epimer toxins (Cl, GTX2) whose amount was 41~57 mol%(27th May to 3rd June in 1996) and 25~56 mole% (29th April to 12th May in 1997), became higher than that of 11-epimer toxins. The toxin compositions in the both samples changed on a daily basis, presumably owing to metabolism of the toxin in the bivalves.