Abstract: An ultrasonic sensor for non-invasive detection of air bubbles in a liquid flowing through a tube comprises two half-housings capable of being assembled together and two mutually opposed ultrasonic transducers, one installed in each of the half-housings. For acoustic coupling of the tube to the two mutually opposed ultrasonic transducers, each half-housing contains a chamber closed off by a flexible membrane and filled with a fluid highly transmissive of sound. The fluid displaced by the tube inserted between the two half-housings forces the membrane up in the form of a bulge on either side of the tube. In this way, an excellent contact is obtained between the chamber filled with sound-transmissive fluid and the tube. The membrane may be embedded into the side walls of the half-housings and cylindrically arched along the same.