Studies of flux motion due to the thermal force resulting in the Nernst effect are complementary to Lorentz-force experiments, because the thermal force is not subjected to spatial redistribution. We have measured the Nernst effect in superconducting epitaxial and polycrystalline Y-Ba-Cu-O films. From our experimental results the transport entropy per unit length of flux line is calculated. Its order of magnitude agrees with the theoretical prediction based on the time-dependent Ginzburg-Landau theory. © 1990 The American Physical Society.