Resistance measurements are reported, carried out on highly textured thin films of YBa 2Cu 3O 7 − gd prepared by co-evaporation of BaF 2, copper and yttrium on (100) SrTiO 3 in a triple e-gun UHV system, adding oxygen during deposition. The polycrystalline films, with average grain size of the order of 10 μm, show a strong preferential orientation, with the c axis perpendicular to the substrate. The resistance measurements in a field parallel to the c axis have been interpreted in terms of a model combining thermally activated flux flow and flux pinning by twin boundaries. The agreement is satisfactory. The field and temperature dependence of the activation energy differs essentially from expressions proposed recently.