Previous studies on aluminum and aluminum-copper alloys have shown that the change of energy loss (△E) under fatigue loading can be divided into two stages. The first stage corresponds to the pinning of dislocations and the rise of △E in the second stage is associated with the occurrence of localized slip regions in the specimen. In order to confirm this viewpoint, torsional fatigue experiments were carried out on aluminum-magnesium alloys containing 0.52, 0.91, 3.46 and 5.15% of magnesium, and the area of the hysteresis loop (stress versus strain) after various stress cycles N was determined, from which the energy loss △E in each cycle was calculated. Results of metallographic observations on the specimen surface after various stress cycles show that for all the alloys used, the change of slip marks exhibits two distinctly different stages. In the first stage, several systems of fine and straight slip lines are often observed, and the slip lines are found to be distributed uniformly over a grain. In the second stage, some of these slip lines are thickened up to form localized slip regions. In general, such localized slip regions occur earlier in alloys with lower magnesium contents (0.52, 0.91%) and under higher applied maximum torsion strains.A comparison was made between the observed changes of the △E-N curves and the occurrence of the localized slip regions. After proper account is taken of the possible superposition of the△E's occurring in the first and second stages, it is shown that the occurrence of localized slip regions in the specimen can give rise to an increase of △E in the later stage of fatigue loading. This agrees with the results previously obtained in the case of aluminum and aluminum-copper alloys.The correlation between the state of the pinning of dislocations in the first stage of fatigue loading and the subsequent occurrence of localized slip regions is discussed.