Abstract: An ultrasonic level sensor (1) for monitoring the level of a liquid in a vessel comprises a main housing (3) having a lower part (5) and an upper part (6). An ultrasonic transducer (19) having an output face (25), through which ultrasonic signals are transmitted and received is housed in a sub-housing (39) located in an engagement recess (17) in the lower part (5). A communicating aperture (24) accommodates ultrasonic signals transmitted by the ultrasonic transducer (19) to and from the liquid surface, the level of which is to be monitored. The sub-housing (39) comprises a cylindrical side wall (40) extending upwardly from a protective element (35) for protecting the ultrasonic transducer (19).