Abstract: To reveal the ultrastructural features of Cymbella subhelvetica Krammer,this study carries out a thorough observation using scanning electron microscope.The observational results demonstrate that C. subhelveticapossesses the following diagnostic features:(1)valves distinctly dorsiventral;(2)distal raphe fissure curved dorsally,proximal raphe ends teardrop not deflected;(3)no apical pore fields;(4)striae uniseriate,slightly radiate except closing to each apex where becoming convergent dorsal striae 9~11/10 μm,ventral striae 9~13/10  μm;(5) Areolae mostly lineate,occasionally Y-shaped,and areolae 30~33/10 μm;(6) 4~7 stigmata.Above characteristics together constitute a unique combination of features that distinguish and identify C. subhelvetica.

Key words: ultrastructure, Cymbella subhelvetica Krammer, stigmata, apical pore fields, Qinghai