A rotational viscometer contains a measuring shaft that can be rotated at a specified rotational speed, a measuring part that is carried by the measuring shaft and that can be lowered into a sample, and a detection unit for determining the angle of torsion of the measuring part relative to the driven part of the measuring shaft. Accordingly, a twistable element is inserted into the measuring shaft, preferably in the section of the measuring shaft close to a motor. The detection unit is moved along with the shaft and determines the relative twist or the angle of torsion (twist) between the shaft part located on the motor side relative to the twistable element and the shaft part located on the measuring-part side. The output signals of the detection unit are fed to an evaluation device as the basis for determining the viscosity of the sample.