The aim of this study is to determine a value for the critical resolved stress for the growth of deformation twins. Loading–unloading tests are performed on extruded magnesium alloy Mg-3Al-1Zn to determine the loads under which twins begin to shrink during unloading. After conversion of the applied stress to mean resolved values, the critical stresses are seen to increase from 6 to 14 MPa as the plastic applied strain is raised from 1 to 6 pct. It is suggested that the “relaxation” dislocations generated to accommodate the twinning strain contribute to building a hard dislocation forest. The effect is analyzed by analogy with accommodation dislocations formed at non-deforming particles.