The population densities, per gram algae, of the toxic benthic dinoflagellate Gambierdiscus toxicus (GTD) on the Hitiaa fringing reef in Tahiti, have been followed since the end of 1976. At that time, during two months, GTD varied between 100 and 800. From mid 1977 to the end of November 1987, it stayed permanently under 50, most of the time under 10. In December 1988 and January 1989, GTD increased from 50 to 13,000 in 3 weeks. It decreased to ca 3,000 one week later and to under 100 one month later. The population densities (OLD) of another benthic dinoflagellate, Ostreopsis lenticularis, frequently associated to G. toxicus in ciguateric areas, but not found to be toxic in French Polynesia, were also high during the same period. The toxin production of G. toxicus was checked at various stages of the bloom.