[en] The evolution of microstructure defects under compression, their mutual interactions at high defect densities and the effect of these phenomena on the hardening were studied in metastable austenitic steel containing 16 wt% Cr, 6 wt% Mn and 9 wt% Ni. At this chemical composition, the estimated martensite start temperature and the stacking fault energy of austenite are about −240 °C and 26 mJ/m"2, respectively. Consequently, the metastable austenite should be thermodynamically stable far below the room temperature and the formation of stacking faults should be retarded. The microstructure analysis, which was performed by means of XRD, ECCI, EBSD and TEM, revealed that the dislocation slip is the dominant deformation mechanism in the initial stages of the deformation process (ε≤0.05). After a critical stress level was reached, stacking faults started to form and widen. In regions with high local stacking fault densities, twinning and the formation of ε-martensite were detected simultaneously. The local deformation mechanism and the related microstructure changes were found to depend on the orientation of the respective grain. In favorably oriented grains, deformation bands consisting of ε-martensite and twins were predominantly formed in the primary slip system. In other grains, the dislocation slip remained the dominant deformation mechanism. The coexistence of faulting, twinning and dislocation slip and the interaction between the dislocations and stacking faults are discussed as the main reasons for the high observed density of microstructure defects and high hardening.