Abstract:In order to clarify the taxonomic issues between Chaetoceros diversus Cleve and C. laevis Leuduger-Fortmorel, their monoclonal strains were established near Nanao Island region, Guangdong Province. The continuous morphology of them was observed during all life stages under light and electron microscopies. The results showed that the extending direction of the thick setae were various in different life stages, so that they are not constant and can't be considered as useful taxonomic features. The D1-D3 sequence of the large subunit encoding ribosomal gene was amplified and analyzed. The results showed that the strains of C. diversus and C. laevis have the coincident sequences except only two differential bases in one strain of C. laevis. Chaetoceros diversus and C. laevis have uniform phylogenetic position in molecular tree, which indicates that they belong to the same species. Therefore, C. laevis is proposed as the synonym of C. diversus.