AbstractA pure culture strain ofCyclotella meneghiniana(KÜTZ.) isolated from the hypereutrophic Bautzen reservoir (March 1980) developed from the typicalCyclotella‐shape to cells with an extremely prolonged pervalvar axis and a small diameter after five months growing under pure culture conditions. This change is interpreted as a consequence of the complete absence of grazing pressure as a selection factor. Since the cell surface/volume ratio showed a singificant increase (3.0) in comparison with the natural population (1.8), it is assumed that the culture strain has also changed with respect to physiological criteria. The conclusion is drawn that investigations of pure cultures for ecological purposes should be restricted to freshly isolated strai