Abstract: Ataxonomic study on 8 species of harmful red tide causative algae was undertaken.Samples were collected from Guangdong coast of South China Sea between October 1997 and April 1998 When red tides occurred in those areas.One spedes of Prymnesiophytes (Phaeocystis of.globosa),which was reported to cause red tide in China for the first time.The others were all dinoflagellates,three of which were toxic.They were 3 species of Gymnodiniales: Gymnodinium mikimotio, Gyrodinium instriatum and Phaepolykrikos hartmanii; 1 species of Dinophysiales: Dinophysis caudata; 2 species of Prorocentrales: Prorocentrum micans and Prorocentrum lima; 1 species of Peridiniales:Alexandrum catenella.