The type material of Synedra acus var. ostenfeldii Willi Krieg. was investigated using light and scanning electron microscopy. The species is one of the few fragilarioid taxa that forms stellate colonies. Its morphology was compared with that of Fragilaria tenera var. lemanensis Druart, Lavigne & M.Robert, another taxon forming star-shaped colonies, on the type slide and in material from a neighbouring lake. The results show both taxa not being conspecific based on valve outline, valve dimensions and presence of spines. Based on the observations, Synedra acus var. ostenfeldii is transferred to the genus Fragilaria as F. ostenfeldii (Willi Krieg.) Van de Vijver, Alexson & Reavie comb. nov. stat. nov. whereas F. tenera var. lemanensis is raised to species level as Fragilaria lemanensis (Druart, Lavigne & M.Robert) Van de Vijver, Ector & Straub stat. nov. Brief notes on their ecology are added.