Mechanial deformation of the YBa₂Cu₃O_7−δ high temperature superconductor under a number of different processing conditions resulted in the formation of and edge dislocations, both having a (001) slip plane. Subsequent high temperature annealing at 900°C resulted in the formation of extrinsic stacking faults with a large separation of the partial dislocations, up to 0.35 μm, suggesting a very low minimum stacking fault energy of 1.2 × 10⁻² J/m². High resolution transmission electron microscopy (HRTEM) in conjunction with image simulations revealed that the stacking faults were comprised of an extra CuO plane between the Ba layers with an offset of b/2. The stacking fault vector of 1/6[031] requires some separation of the Burgers vectors into the c-axis direction. A model in which [010] separates into 1/6[031] + 1/[031] is consistent with the observed stacking faults.