In a method for operating an ultrasonic sensor in duplex mode, a transmission trigger for triggering an acoustic transmitted signal is transmitted from a control device for controlling the ultrasonic sensor to the ultrasonic sensor via a first duplex channel at a multiple of a resonance frequency of an ultrasonic converter in the ultrasonic sensor. The transmission trigger is subsequently divided down to the resonance frequency of the converter in the ultrasonic sensor. At least one acoustic received signal which corresponds to the acoustic transmitted signal and which is delayed with respect to the reflection at an object due to the acoustic propagation time is converted by the ultrasonic sensor, using the converter, to an electrical received signal which is transmitted from the ultrasonic sensor to the control via a second duplex channel at the resonance frequency of the ultrasonic converter in the ultrasonic sensor.