  Anisotropic penetration of magnetic flux in MgB2 films grown on vicinal
sapphire substrates is investigated using magneto-optical imaging. Regular
penetration above 10 K proceeds more easily along the substrate surface steps,
anisotropy of the critical current being 6%. At lower temperatures the
penetration occurs via abrupt dendritic avalanches that preferentially
propagate {\em perpendicular} to the surface steps. This inverse anisotropy in
the penetration pattern becomes dramatic very close to 10 K where all flux
avalanches propagate in the strongest-pinning direction. The observed behavior
is fully explained using a thermomagnetic model of the dendritic instability.
