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Results are reported of an investigation of the electrical properties of Ge films evaporated on Duran glass at different substrate temperatures TS. The resistivity measurements allow to distinguish between amorphous and polycrystalline films on account of a steep resistivity step of four or seven orders of magnitude at 294 or 80 K lattice temperature, respectively as found at TST = 135°C. This step is due to the fact that with the relatively high evaporation rate used, grains of only equal size are performed, and that with decreasing TS at TST all grains fall below a critical grain size. By the analysis of the lattice temperature dependence of the resistivity of the polycrystalline films, making use of the mean free path of carriers between two collisions at grain boundaries, the critical grain size was found to be about an elementary cell.