Abstract The acoustic emission (AE) technique was used to monitor cyclic tensile and tension-compression tests of the prospective Mg-based light alloy AS21X. The dependence of AE on microstructure was investigated. It was shown that the AE response depends significantly on the grain size and the maximum stress amplitude of cyclic loading. An anisotropy in the mode of loading is also seen, with increased AE activity under compressive loading. The results are explained using the AE analysis that correlates the stress–strain curves with possible deformation mechanisms, i.e. an interplay of the dislocation glide and deformation twinning.