The influence of different loading conditions on the microstructural development of extruded magnesium alloy AZ31 was investigated by optical microscopy and electron backscattered diffraction. Extruded magnesium profiles exhibit a significant asymmetry in the mechanical properties, due to the low activation energy of the extension twinning system {10 (1) over bar2} < 10 (1) over bar1 >, when compressing along the extrusion direction. For the analyses of this twinning system, compression tests with different applied strains 0.4 <= epsilon <= 11% were performed for two extrusion products exhibiting different microstructures. The main deformation mechanisms during cyclic loading are the formation of extension twins during compression and the detwinning during subsequent tensile loading. The strain-controlled fatigue tests were carried out with applied strain amplitudes 0.3 <= epsilon(A) <= 5%. The tests were stopped at characteristic numbers of cycles N in the tensile or compression maximum of the hysteresis loop. The microstructural investigations deliver information about the type of twinning and the size, shape, local distribution, and volume fraction of twins as a function of the plastic deformation. These results will be discussed with regard to the microstructure of the initial state material and to the applied load.