  We examine Shapiro steps for vortex lattices interacting with periodic
pinning arrays driven by AC and DC currents. The vortex flow occurs by the
motion of the interstitial vortices through the periodic potential generated by
the vortices that remain pinned at the pinning sites. Shapiro steps are
observed for fields B_{\phi} < B < 2.25B_{\phi} with the most pronouced steps
occuring for fields where the interstitial vortex lattice has a high degree of
symmetry. The widths of the phase-locked current steps as a function of the
magnitude of the AC driving are found to follow a Bessel function in agreement
with theory.
