Abstract
Water samples taken from Loch Leven, Scotland, contained unicellular centric diatoms dominated by a taxon we called Stephanodiscus hantzschii Grunow. It existed as several morphotypes, and these are described and compared with the type material of S. hantzschii from the Rabenhorst exsiccata 1104. The morphological features of material subjected to experimental nutrient enrichment are also briefly described. Two very distinct morphotypes and an additional three morphotypes, with more or less constant features of valve pattern have been distinguished. In addition, the presence of hemispherical valves made it possible to describe stages in the life cycle of this very common centric diatom, and an amended description of S. hantzschii is given.