Abstract: A pipe wall thickness gauge including an acoustic signal generator and sensor in a housing mounted in a base adapted for underwater use and continuous monitoring of the wall thickness of a pipe, such as freshly extruded plastic pipe. The gauge includes a base adapted to surround the pipe, the base consisting of at least two base sections each having at least one pipe positioning member with pipe centering elements at the tip thereof; at least one of the pipe positioning members including a mounting means for the housing of the acoustic signal generator and sensor. Preferably, adjusting means are included to move the pipe positioning member inwardly or outwardly to accomodate different diameter pipes and to vary, within limits, the position and direction of the acoustic signal generator and sensor. Various other features are included in the preferred form of the present invention.