Two clones of Protogonyaulax cohorticula were isolated from the Gulf of Thailand. The extracts of these clones killed mice with typical signs of paralytic shellfish poisoning. The toxicities corresponded to those of strongly toxic clones of P. tamarensis. In the HPLC and electrophoretic analyses, gonyautoxins and saxitoxin were detected. About 80% of the toxins consisted of gonyautoxin I. These results show that P. cohorticula is a toxic species of Protogonyaulax and that it is at least one of the causative organisms of paralytic shellfish poisoning in Thailand.