Small diatoms belonging to the genus Fallacia often constitute a diverse component of the benthic protist communities of brackish and marine sandy sediments worldwide. However, the systematics of these organisms, and especially the epipsammic representatives, is incompletely known, and this seriously hampers the interpretation of ecological and biogeographical information. In this study, we give a detailed morphological and, where possible, cytological description of 10 Fallacia species from the Westerschelde estuary (The Netherlands); they are extensively compared to similar species. Autecological and biogeographical information on the considered taxa is provided as well. Four new species, Fallacia arenaria Sabbe et Vyverman, F. margino-punctata Sabbe et Vyverman, F. pulchella Sabbe et Muylaert, and F. scaldensis Sabbe et Muylaert, are described, and two new combinations, Fallacia escorialis (Simonsen) Sabbe et Vyverman and F. wuestii (Simonsen) Sabbe et Muylaert, are proposed. In addition, the type materials of several problematic taxa have been investigated. Examination of slides from other European and North American localities reveals that many small Fallacia species (including some of the newly described species) have been overlooked in previous studies. The large morphological heterogeneity in Fallacia notwithstanding, cytological and morphological evidence suggests that Fallacia forms a natural, monophyletic taxon. uniquely characterized by the possession of a porous conopeum, lateral (often depressed) sterna, and an H-shaped plastid. The main morphological characters used for species identification within Fallacia are evaluated.