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 interstitial 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. [S0163-1829(99)51434-3].