3 Chinain, M. Revel, T. Faust, M. A. Holmes, M. J. Ung, A. Pauillac, S. 2001 In : Harmful Algal Blooms, Hallegraeff G.M., Blackburn S.I., Bolch C.J. and Lewis R.J. , Eds). Intergovernmental Oceanographic Commission of UNESCO, pp 226-229 2001 The benthic dinoflagellate Gambierdiscus is regarded as the primary causative agent of Ciguatera Fish Poisoning, a disease prevalent in the Pacific regions, the Carribean and the Indian Ocean. Six distinct species within this genus, most of them being toxic, have been described so far on the basis of morphological criteria. The genetic characterization of various strains of G. toxicus has also been addressed in recent studies. In the present work, we investigated the usefulness of the rRNA genes for the molecular characterization and classification of Gambierdiscus isolated distributed in 5 of the 6 species described to date.Sequences of the 5.8S+ITS rDNA and the LSU rDNA D8-D10 regions of 11 Polynesian isolates were compared for phylogenetic analysis. Both regions proved to be useful biogeographical markers, as a grouping of these isolates according to their geographic origin (northern versus southern islands) was globally observed. To investigate the potential intesrest of the LSU rDNA D8-D10 regions in Gambierdiscus systematics, sequences of 8 isolates from distinct geographic origins distributed among 5 of the 6 morphospecies described to date, were also compared. Four molecular types corresponding to G. australes, G. polynesiensis, G. yasumotoi and G. toxicus/G. pacificus lineages could be distnguished. These results suggest that, within this genus, genetic data are fairly consistent with morphological classification by SEM microscopy. They also indicate that distinguishing species of Gambierdiscus can combine both molecular and traditional morphological criteria.