  We study the interactions of the vortex lattice with a periodic square array
of holes in a superconducting Nb film using magnetic and resistive
measurements. Three different temperature regions have been observed. They are
governed by interplay between vortex-vortex interactions and pinning by holes.
At low temperatures flux jumps suppress the commensurability anomalies. In all
measurements a peak has been observed close to commensurate states 5-7. The
Cole-Cole plot reveals significant changes in the flux penetration mechanism at
this point.
