Electron diffraction is extremely useful in studying membrane preparations in microscopic quantities. Although certain biological membranes, especially those occured in natural lemella form, have been subject to X-ray diffraction studies, so far no electron diffraction study of wet membranes had been reported, apart from a preliminary attempt made in this laboratory.! The reasons had been: 1) The inability to keep the specimen in its natural state during observation, and 2) The naturally occured lemella of membranes such as nerve myelin, retina rod and chloroplast are usually too thick for the electron beam to penetrate.We report here our preliminary results on an electron diffraction study of the disc membranes of whole wet retina rod outer segments, using the 1 Mev electron microscope at the U.S. Steel Research Center at Monroeville, Pa., with a hydration stage built by this laboratory for that microscope.