The Volga, which is the largest river in Europe (3690 km long), flows from the north (Tver' region) to the south (Caspian Sea), and its extensive basin (1380 km2) includes very different biotopes. Thus, analysis ofthe occurrence of Paramecium species along this large river basin may significantly enhance our understanding of species distribution according to temperature regime, food richness and other possible factors. The present paper concerns the occurrence of species of the P. aurelia complex in the sampling areas of the Upper Volga River, and a comparison with the occurrence of species of the P. aurelia complex in the Lower Volga region. In the Upper Volga basin, P. biaurelia was the most abundant among species of the complex recorded (among P. triaurelia, P. decaurelia, P. dodecaurelia), in the Lower Volga region eight species of the complex were recorded (P. primaurelia, P. biaurelia, P. triaurelia, P. pentaurelia, P. sexaurelia, P. septaurelia, P. novaurelia, P. decaurelia).