This work presents a new ultrasonic fatigue testing device for studying the initiation and propagation mechanisms of internal microstructurally short fatigue cracks using in situ synchrotron tomography. Its principle is described as well as the method used for automatically detecting crack initiation and its subsequent growth. To promote internal crack initiation, specimens containing internal casting defects were tested between the high cycle and very high cycle fatigue regimes (10 7 -10 9 cycles). Preliminary results show the ability of this new device to initiate an internal microstructurally short crack in a reasonable testing time and monitor its growth.