This study looked at toxicity of Mediterranean mussels, Mytilus galloprovincialis, which had accumulated paralytic shellfish toxins (PST) from early March to late May 2005 at Jinhae Bay, Korea. Alexandrium sp. was observed in low densities (< 1,000 cells/L) at the beginning of the study in March, increased rapidly in April, declined rapidly and disappeared in May. Although low densities of Alexandrium sp. were observed in March, mussel toxicity exceeded regulation level ($80{mu}g$ STXeq. /100 g). Peak PSP (Paralytic Shellfish Poisoning) toxicity in the mussels occurred during high Alexandrium sp. cell densities in April. Mussels toxicity decreased with decline of Alexandrium sp. cell density. Major toxin components identified were $GTX_1$, $GTX_4$, followed by $C_1$, $C_2$, $GTX_2$, $GTX_3$ and neoSTX. Trace or sporadic toxin components were STX, $GTX_5$, $dcGTX_2$, $dcGTX_3$ and dcSTX. Toxin component analysis from the middle to end of the study showed that $11{eta}$-epimers ($GTX_{3,4}$, $C_2$) were converted into $11{alpha}$-epimers ($GTX_{1,2}$, $C_1$) and started to determine STX.