In the previous report1) we have described the occurrence and general features of a new type of shellfish poisoning. Although no red tide or fish kill was sighted preceding to or during the infestion period, signs such as seasonality, regionality, and depth-dependence of shellfish toxicity strongly suggested the implication of a dietary plankton in the poisoning. The symptoms in human cases were mainly gastrointestinal disorders and lacked paralytic symptoms, denying the possibility that Protogonyaulax or Gymnodinium breve were the cause of the poisoning. Consequently our aim was set at the identification of the organism which would transmit the toxin to shellfish. Our first approach was to find out an organism whose occurrence in water would go in parallel with the toxicity of shellfish. In the second experiment planktons were separated into several fractions according to their sizes on sieves with dif ferent mesh sizes. Testing of such size-selected samples gave the approximate size of the causa tive organism. Further effort was made to find out an organism of which distribution among the sieved fractions coincided with the distribution of toxi city. Finally, toxins from the plankton sample and mussels were compared by chromatography in order to make confirmation of their identity. Since all the results unequivocally indicate that a dinoflagellate Dinophysis fortii is the causative organism, we propose to name the toxin dinophysi stoxin. Also a term "diarrhetic shellfish poisoning" will be used hereafter for the poisoning as diarrhea is the most predominant symptom over others.