Gambierdiscus toxicus gen. et sp. nov. is described. This is a toxin producing dinoflagellate which was collected in a ciguatera-endemic area. Observations were made using both a light microscope with or without phase-contrast and a scanning electron microscope. This species is distincdve in its antero-posterior compressed body shape, with a circular narrow deep cingulum and a deep hollow sulcus. The theca consists of 33 plates: Po, 3', Oa, 7", 6c, 8s, 6"', 1p, 1"". The plate pattern of this species is characteristic in having extremely reduced precingulars 1" and 7", which are situated between apical 1' and the sulcal excavation and are clearly separate from apical 2" and 4". These features and others indicate that this genus should be assigned to the Heteraulacaceae family. This species is always sessile and attaches to dead coral and seaweed, especially brown alga, Turbinaria ornata J. AGARDH. A large number of this species were taken around the Gambier Islands, French Polynesia, in May 1975.