An apparatus for the observation of low-energy electron diffraction patterns on a fluorescent screen is described. The diffracted electrons pass through a pair of grids and then are accelerated by a potential difference of the order of 103 v to excite the fluorescent screen. The equipment is designed for high vacuum, and pressures below 2×10-10 mm Hg have been maintained. Different gases can be introduced at low controlled pressures (10-9 to 10-7 mm Hg), and diffraction patterns produced by adsorbed layers can be photographed and studied. Typical diffraction patterns from a clean (111) face of a nickel crystal and from oxygen adsorbed upon this face are reproduced here.