  Thermally-activated flux flow (TAFF) resistivity above the irreversibility
field is reported for two different c-axis textured MgB_2 superconducting
films. Transport measurements at different perpendicular magnetic fields 0 < B
< 9 T and temperatures from 5 to 40K reveal TAFF Ohmic resistivity described by
the Arrhenius law with the quadratic field dependence of the activation energy.
Our transport measurements on bulk MgB_2 ceramic samples also show the TAFF
behavior, but do not show the quadratic field dependence of U(T,B). We explain
our results in terms of thermally-activated drift of pre-existing quenched
dislocations in the vortex lattice. Our results indicate that thermal
fluctuations can be essential in determining the irreversibility field in MgB2
though to a much lesser extent than in high-temperature superconductors.
