Despite being one of the dominant groups in the diatom flora of our rivers and lakes, the correct taxonomic identity of several Fragilaria Lyngb. taxa (formerly in the genus Synedra Ehrenb.) is often blurred by insufficient knowledge of the different morphological types. The present paper illustrates and discusses several former Synedra taxa using both light and scanning electron microscope observations. Based on the morphological analysis, Synedra deformis W.Sm. is transferred to the genus Fragilaria as Fragilaria deformis (W.Sm.) Van de Vijver & Ector, comb. nov. A second Synedra species, S. vaucheriae var. deformis Grunow, is likewise investigated but proved a later synonym of F. pectinalis (O.F.Müll.) Lyngb. The morphological results are compared with the (type) material of several similar Fragilaria species such as Fragilaria joachimii Kahlert, F. vaucheriae (Kütz.) J.B.Petersen, F. pectinalis and F. candidagilae Almeida, C.Delgado, Novais & S.Blanco. Notes on the ecology of the different species are added.