Abstract: An ultrasonic testing system for testing the circumferential girth welds of a tank for defects utilizes a probe trolley to which six probes are attached. The probe trolley is clamped to a drive unit which runs upon a track that is removably attached to the interior surface of the tank. The track is positioned so that the probe trolley travels over the weld as the drive unit negotiates the track. Coupling fluid is supplied to the probe trolley so that a layer of coupling fluid exists between the probes and the tank interior surface adjacent to the weld. Ultrasonic beams emitted by the probe travel through the coupling fluid, into the tank wall and weld and are reflected. Data from the probes is directed to a processor where it is analyzed, displayed and stored.