We report oscillations in diffracted electron intensities during ion bombardment of Si(100) by 200- and 250-eV Xe, both alone and with sequential and simultaneous epitaxy. Analysis of the phase and frequency of the oscillations shows that, to first order, ion bombardment undoes'' previous epitaxy and cancels or partially cancels simultaneous deposition. Surprisingly, the phase relationship of growth and sputtering is both antisymmetric {ital and} linear, indicating that the ion-induced oscillations are dominated by simple, vacancy-mediated, layer-by-layer sputtering.