Magnesium alloys, on account of their lightweight, find useful applications in the automotive sector. During service, they experience very high number of fatigue cycles. Therefore, the understanding of their long life fatigue behavior becomes extremely important. This is possible by using ultrasonic fatigue testing, which is the only feasible way of doing it. In this study, the two such alloys viz. AE42 and AM60 has been investigated for their long life fatigue characteristics under fully reversed loading conditions, using a piezoelectric fatigue testing machine operating at a frequency of 20 kHz. The S-N data does not reach a horizontal asymptote at 107 cycles in either of the alloys. However, the alloy AM60 seems to show a fatigue limit at about at 109 cycles. The fractures examined by scanning electron microscopy (SEM) were found to be brittle in character. In very high cycle fatigue conditions, the crack was found to initiate from the specimen subsurface.