Seventeen species of a potential harmful microalgae were encountered during a survey of phytoplankton diversity project from the Gulf of Thailand. Fifteen are dinoflagellate and the rest are diatom. Seven species (Rhizosolenia styliformis, Trichodesmium erythraeum, Ceratium furca, C. trichoceros, Gymnodinium sanguineum, Noctiluca scintillans and Prorocentrum micans) can be the causative organisms for red tide phenomena. Four (Alexandrium minutum, A. tamarense, A. tamiyavanichi and Gymnodinium cf. catenatum are PSP-causing, three (Dinophysis caudata, D. miles and D. mitra) are DSP-causing and the rest (Amphidinium caterae, A. klebsii and Prorocentrum lima) are ciguatera-causing organisms. Even the red tide is sometimes occurred in the Gulf of Thailand but no incident of toxic shellfish poisoning was reported since 1983.