Abstract
Diatoma polonica, a species first observed in samples from the Western Carpathians river systems in southern Poland, is described as a species new to science. The new species status resulted from the fact that the dimensions of cells found did not fit any of the diagnoses given in the literature for the European Diatoma taxa established so far. The genus is rather species-poor, even when the entire Holarctic flora is taken into account. The new species is morphologically closest to Diatoma moniliformis Kützing, particularly with respect to the vague resemblance of the valve outlines and the presence of a rimportula in each valve pole. Further, more or less similar taxa are D. tenuis Agardh, D. problematica Lange-Bertalot and D. mesodon (Ehrenberg) Kützing, all distinguished, in addition to other specific characteristics, by the rimoportulae regularly present at only one valve pole. Characteristics of D. polonica are described and compared with those of other species based on light and scanning electron microscopy.