We consider the precise role, in the biogenesis of ciguatera, played by the dinoflagellate Gambierdiscus toxicus which had been previously isolated in the Gambier Islands (French Polynesia) from toxic biodetritus covering dead corals. We studied the characteristics of the toxicity and its quantitative correlation with dinoflagellate numbers, assaying various samples of non-fractionated biodetritus, a sample of biodetritus fractionated according to particles size, and samples of G. toxicus cultured cells. The toxic substances, isolated in vivo and in vitro, have almost the same biochemical and biological properties as the reference ciguateric toxins. The direct and reproducible relationship between the number of G. toxicus cells and the toxins concentration in the biodetritus, and the capacity of the monoalgal G. toxicus cultured cells to produce the ciguatera toxin complex, confirm the dinoflagellate as the responsible agent of the phenomenon in French Polynesia. The distribution of this dinoflagellate in other endemic areas of the Pacific and the West Indies, provides a presumptive argument for a common worlwide origin for ciguatera.