A simple electrostatic device has been constructed which, when inserted in the optical system of an electron microscope, functions as an absorbing phase plate. Its operation depends on the central portion of a thin poorly conducting thread generating a stable potential under the influence of the electron beam and creating a particular form of electric field. An electron interference technique is employed to study the stabilizing mechanism and to develop a method for achieving the required magnitude of potential. The performance of this device is gauged by optical diffraction of electron micrographs of a thin carbon film; its application is illustrated by examining some negatively stained biological specimens. The results indicate that such an ‘electrostatic phase plate’ can provide significant improvements in contrast and signal/noise ratio over normal bright field images without loss in resolution.