Abstract Contamination of yessotoxin (YTX) and its analogs in shellfish has occurred worldwide and has seriously damaged shellfish industries. One of the sources of YTX has been identified the dinoflagellate Protoceratium reticulatum. A new analog of YTX, 45,46,47-trinorhomoYTX, was isolated from cultures of the dinoflagellate P. reticulatum collected at Yamada Bay, Iwate in Japan. Its structure was determined by analysis of MS and NMR experiments. This is the first isolation and confirmation of a homoYTX analog in Japan.