A remarkably reversible slip-unslip process has been observed on low strain amplitude fatigue hardened copper single crystals. Slip lines formed on an electropolished surface in one half cycle almost vanish on the subsequentreverse half-cycle, then reappear on the next forward stroke. This process has been observed at least 300 μ below the original surface shortly after saturation. The dislocation structure was composed of dense dipole-monopole mats elongated on the primary slip plane and it is suggested that the reversibility results from interactions between these mats and their surrounding elastic matrix.