AbstractElectro‐optic and rheological measurements were carried out on PTFE‐fibrilles. The crystalline fibrilles had an axial ratio of about 100 and a thickness of 200 Å. The dispersions were stabilized by ionic Perfluorosurfactants and could be kept for months without precipitation. — The rotational diffusion constant of the fibrilles was determined as a function of the volume fraction. Both electric birefringence and rheological measurements gave identical results for the orientation times. In the dilute and semidilute concentration range the rotational diffusion constant was the same and did not change at the overlap concentration. The shear modulus was found to increase in the semidilute concentration with the square of the volume fraction of the PTFE fibrilles. — Electric birefringence measurements with AC‐pulses showed a remarkable result. The stationary birefringence changed sign as a function of the frequency. It is concluded that this phenomenon is related to the adsorption/desorption equilibrium of the surfactant ions on the PTFE fibrilles. The equilibrium is shifted by the electric field. These conclusions could be confirmed by light scattering measurements in