A sonde in a well logging system of the present invention includes caliper means providing a diameter signal corresponding to the diameter of a borehole traversing an earth formation. First and second transducers which, when so controlled, transmit an ultrasonic acoustical pulse into fluid flowing in the borehole or provides an electrical pulse in accordance with a received acoustical pulse from the fluid. A control network controls both transducers so that during one part of a measurement the first transducer transmits an acoustical pulse into the fluid while the second transducer receives an acoustical pulse after it has passed through the fluid. During another part of the measurement the second transducer is controlled to transmit an acoustical pulse into the fluid while the first transducer is controlled to receive the acoustical pulse after it has passed through the fluid and provides a signal in accordance with the received acoustical pulse. A signal circuit connected to both transducers provides a travel time signal corresponding to the travel times of the acoustical pulses for both parts of the measurement. A conventional well logging cable carries control signals from the surface to the control network and the travel time signal and the diameter signal to surface electronics. The surface electronics includes processing apparatus which provides the control signal to the cable and which derives the flow rate of the fluid flowing in the borehole in accordance with the travel time signal and the diameter signal.