An ultrasonic sensor is disclosed for performing a propagation-time measurement in a liquid. The ultrasonic sensor includes an ultrasound transmitter and a deflecting element made of plastic, into which deflecting element the ultrasound pulses emitted by the ultrasound transmitter are conducted, which ultrasound pulses are deflected on a boundary surface within the deflecting element by reflection. A hermetically sealed air bag is arranged inside the deflecting element. A wall of the air bag forms the plastic/air boundary surface for reflecting the ultrasound pulses. The ultrasonic sensor may provide improved functional capability of the deflecting element over a long time period, while being economical to produce.