The invention relates to a method for determining the filling level of a medium in a container. A mixer that has at least one mixer blade and a mixer frequency (f.sub.<1>R) is disposed in the container. Measuring signals are emitted at a predetermined measuring frequency (f.sub.<1>M) towards the medium. The measuring signals reflected on the surface of the medium are received and evaluated by a run time method. The aim of the invention is to provide a device which allows for a reliable measurement of the filling level of a medium in a container provided with a mixer. To this end, the measuring frequency (f.sub.<1>M) of the measuring signal having the mixer frequency (f.sub.<1>R) of the mixer is synchronized (f.sub.<1>M=n f.sub.<1>Rwith n .epsilon. N) and that the measuring frequency (f.sub.<1>M) is shifted by a defined phase (?) with respect to the mixer frequency (f.sub.<1>R).