ABSTRACT The analysis of electron micrographs by optical diffraction was introduced recently by Klug & Berger (1964). Their experiments were conducted with a special diffractometer designed for use with diffracting masks up to several inches in diameter. A method is described here for using a conventional light microscope as an optical diffractometer which can accept masks up to 5 mm in diameter. A-100 μ electron-microscope aperture is used as a pinhole source of illumination, and the micrograph to be studied is introduced above the objective. The diffraction pattern produced by the micrograph appears in the usual image plane of the microscope within the eyepiece.