In an investigation of the differences and similarities within the species Theodoxus fluviatilis, we analysed 11 brackish water populations from the Baltic Sea and 8 freshwater populations from northern Germany. T. fluviatilis from the two habitats are primarily distinguished by different habitat preferences. We have never observed the migration or colonization of rivers by brackish water animals or vice versa. While mostly stones and wood serve as typical substrata in freshwater, we found both stones and plants (Fucus, Potamogeton, Zostera) as habitats in brackish water environments. In an analysis of morphology (shell size, operculum, radula), we found no significant differences between brackish and freshwater populations of T. fluviatilis in the Baltic Sea area. The variability of shell morphology and ecological behaviour of T. fluviatilis within and/or between brackish and freshwater populations is most likely based on different habitat conditions and demonstrates the large phenotypic plasticity of the species.