Shock-induced lattice faults have been studied in stainless steel subjected to explosive deformation pressure pulses in the range 120 to 425 kilobars. It has been found that the deformation faults lie on {111} lattice planes. Selected area diffraction also indicates that the faults possess an imperfect but pronounced twin orientation with respect to the matrix. The fault density deduced from electron micrographs increases almost linearly with peak shock pressure and exhibits a direct correlation with microhardness. Evidence is also produced to show that there is a transition from single stacking faults to imperfect twin faults with increasing shock pressure.