Abstract: An apparatus for detecting gas bubbles, in a liquid flow, comprises a one-piece housing (1) of a material that is transmissive to ultrasound, a liquid passage (2) extending through the housing (1), inlet and outlet means on the housing (1) for connecting the liquid passage (2) to the liquid flow, and ultrasonic transducer means (3, 4) on the housing on each side of the liquid passage (2), one transducer means being arranged as sender and the other transducer means being arranged as receiver, with the liquid passage (2) positioned in the sound transmission path between the sender and the receiver, whereby inhomogeneities in the liquid flow can be detected based on the emitted ultrasound energy that is received by the receiver. The housing (1) has two opposed recesses (6, 7), one on the sender side of the liquid passage (2) and one on the receiver side thereof, which recesses in a direction perpendicular to the sound transmission path as well as to the liquid passage.