In the present study a search was made for a platkton which transmit a fat-solublr toxin to shellfish and thus lead to a new type of shellfish poisoning. A regular monitoring on phytoplanktons at 10 m and 24 m depths of Okkirai Bay revealed that the occurrence of a dinoflagellate Dinophysis fortii paralleled well, both in time and quantity, with the variation of mussel toxicity. When sea water samples were passed through sieves of varying mesh sizes, toxicity was detectable only in a sample trapped in a 40-95 μm fraction. D. fortii was not only concentrated exclusively in this fraction but also its abundance was proportional to the toxicity levels of the plankton samples. Possibility of other organism being toxic was eliminated by commparing the abundance of each species in sieved fractions of different sources. The plankton toxin was indistinguishable from the mussel toxin in both gel permeation chromatography and partition chromatography. All the above results unequivocally support that D. fortii is responsible for inducing toxicity in shellfish. It was proposed to name the toxin dinophysistoxin and the poisoning diarrhetic shellfish poisoning.