The switching effect induced by an electric field and pressure has been investigated in thin (∼0.5 &mgr;m) films of polydiphenylenephthalide. It has been shown that the conductivity of a polymer film is affected by highly conductive filament formation. Using a liquid crystal, we were able to visualize such filaments and to determine their parameters: size, conductivity, and surface density. The low‐frequency current fluctuations in thin polymer films have also been measured. The rapid growth of current fluctuations near the switching point has been found. The results of this study are evidence for the filament model. ©1995 American Institute of Physics.