Abstract: The present invention relates to a system for the nondestructive testing of the thickness of boiler tubes. The system comprises a probe assembly which includes a mounting unit for mounting the assembly in a mud or water drum of a boiler at the location of a boiler tube to be tested. The probe assembly is comprised of an ultrasonic transducer for transmitting and receiving ultrasonic signals. The probe assembly further includes a motor drive connected to the transducer. A control unit is provided which includes a motor control portion connected to the motor drive to selectively position the transducer at a plurality of locations in the boiler tube to be tested. The control unit also includes an ultrasonic drive for producing an ultrasonic pulse for driving said transducer means. A water control is connected to the control unit, a source of water supply, a drain, and the probe assembly.