Abstract: To permit this flowmeter to automatically operate either on one measurement principle or on another depending on a selectable foreign-particle content of the fluid, it has a first pair of ultrasonic transducers, one of which is operated as a transmitting transducer and the other as a receiving transducer or vice versa, and which are mounted on the outside surface of a fluid-carrying pipe and separated by a given distance in a direction parallel to the axis of the pipe, with the angle between the direction of the ultrasound and the pipe axis differing from 90.degree.. There is provided a second pair of ultrasonic transducers which are operated simultaneously as transmitting transducers or receiving transducers and are mounted on the outside surface of the pipe and separated by a given distance in a direction parallel to the axis of the pipe, the angle between the direction of the ultrasound and the pipe axis being less than or equal to 90.degree..