An ultrasonic transducer has a transducer housing having an ultrasound window, a housing tube and a housing flange, a transducer holder for holding the transducer housing, a transducer element with an ultrasonic flow rate measuring transducer that is positioned at an end of the housing tube of the transducer, and an opposing flange clamping the housing flange of the transducer housing to a holder flange of the transducer holder together with retraining screws and lock nuts. A side of the housing flange of the transducer housing facing the opposing flange has an outer contour which enables bracing of the opposing flange against the housing flange without tilting or bending of the housing flange.