Marine benthic dinoflagellates have been collected from macroalgae of Truong Sa archipelago. Twenty-three species of benthic dinoflagellates were registerred from brown algae (Dictyota and Padina)
red algae (Amphiroa, Galaxaura, Heminthocladia, Laurencia, and Liagora)
green algae (Halimeda and Caulerpa). They are Prorocentrum arenarium, P. concavum, P. emarginatum, P. lima, P. rhathymum, P. sculptile, P. sp., P. tropicalis, Coolia monotis, C. tropicalis, C. sp., Gambierdiscus pacificus, G. toxicus, Ostreopsis labens, O. lenticularis, O. marinus, O. ovata, O. siamensis,Sinophysis canaliculata, S. microcephala, Bysmatrum caponii, B. granulosum and Peridinium quinquecome. Cell densities greatly varied in spatial and depending on substrata and sampling depth. Cell densities of benthic dinoflagellates were high on macroalga Padina, 2,043 cells/g FW in Sinh Ton island and 1,312 cells/g FW in Rac Truong Sa island. Cell densities were not high in Son Ca, Nam Yet and Da Tay islands. The toxic species, Gambierdiscus toxicus and G. pacificus, causing of ciguatera fish poisoning (CFP) were found. Cell densities of these species were 126 cells/g FW of Laurencia in Rdc Truong Sa island and 120 cells/g FW of Galaxaura in Sinh Ton island.