  We study the interactions of the vortex lattice with a periodic square array
of holes in a superconducting Nb film. Using high resolution magnetic field
measurements of electrical losses, extremely narrow states with a magnetic
field width reaching 1% of the matching field value are found at the
commensurate points. They are accompanied by pronounced harmonic generation in
response to ac modulation of the magnetic field or current. We relate these
sharp anomalies to a locked commensurate state of the Mott insulator type.
