The diagnostic characteristics of the genera Achnanthidium and Psammothidium, the latter genus recently separated from other species of Achnanthes sensu lato, of the family Achnanthidiaceae D.G. Mann have been evaluated on the basis of a morphological study of eleven common species found in rivers and streams of Luxembourg. In light of these observations – supplemented by a literature review – it is concluded that the two genera cannot be unambiguously defined on the basis of morphological criteria. Furthermore, no ontogenetic or phylogenetic studies support any of the discriminating characteristics. On the basis of these results, Achnanthes bioretii, A. daonensis, A. austriaca var. helvetica, A. lauenburgiana and Navicula subatomoides have been transferred to Achnanthidium.