The invention relates to a device (7) for detecting abrasive wear on components (21, 22) of water turbines (2), comprising a wear body (9) and a measuring device (13) for detecting wear on the wear body (9). The device is characterized in that the wear body (9) is arranged in the current flowing through the water turbine (2), the wear body is designed as a sonotrode (9) and is connected to an ultrasonic generator (11), and the measuring device (13) is designed to measure the natural frequency of the sonotrode (9).