  Shapiro voltage steps at voltages V_n=nV_0 (n integer) have been observed in
the voltage-current characteristics of a superconducting film with a square
lattice of perforating microholes (antidots)in the presence of radiofrequent
radiation. These equidistant steps appear at the second matching field H_2 when
the flow of the interstitial vortex lattice in the periodic potential created
by the antidots and the vortices trapped by them, is in phase with the applied
rf frequency. Therefore, the observation of Shapiro steps clearly reveals the
presence of mobile intersitial vortices in superconducting films with regular
pinning arrays. The interstitial vortices, moved by the driving current,
coexist with immobile vortices strongly pinned at the antidots.
