  We observed mode-locking (ML) of rf-dc driven vortex arrays in a
superconducting weak pinning a-NbGe film. The ML voltage shows the expected
scaling $V\propto f\sqrt{B}$ with $f$ the rf-frequency and $B$ the magnetic
field. For large dc-velocity (corresponding to a large ML frequency), the ML
current step width exhibits a squared Bessel function dependence on the
rf-amplitude as predicted for ML of a lattice moving elastically through a
random potential.
