In 2013, paralytic shellfish poisoning (PSP ) occurred in Kesennuma Bay, northern Japan, after an interval of 24 years. From April to early May, Alexandrium tamarense increased in the inner part of the bay (up to 136,200 cells/ L), expanded to the whole of the bay, and caused PSP. In contrast, A. catenella increased in early September (up to 1,310 cells/L) in the center of the bay, but did not cause PSP. Resting cysts of A. catenella/tamarense ranged from 30 to 6,383 cysts/cm 3, and dominated in the inner part of the bay. A meta-analysis of temporal trends in the density of A. catenella/tamarense cysts in Kesennuma Bay revealed that the cysts in the surface bottom sediment drastically increased after the Great East Japan Earthquake. High PSP toxicity was observed in four bivalves, Chlamys farreri (130 MU/g), Mytilus galloprovincialis (110 MU/g), Ruditapes philippinarum (74 MU/ g ), and Mizuhopecten yessoensis ( 66 MU/g).