Abstract: Apparatus for measuring the thickness of the walls of liquid-bearing pipelines provides a transmitter and a receiver of ultrasonic waves supported to move down the pipelines in spaced relationship to the walls, and provides means associated with the receiver for detecting the reflections of the transmitted waves from both the inner and the outer walls of the pipeline and for recording the time interval between such detections as a measure of pipe thickness.Another aspect of the invention provides, for measuring the thickness of the walls of gas-bearing pipelines of providing a liquid filled tire which rolls on the pipeline wall as the detecting apparatus travels therealong and which tire and its contained liquid contain the ultrasonic transmitter and receiver.