Ribosomes crystallize on endoplasmic reticulum membranes in oocytes of the southern Italian lizard, Lacerta sicula, during winter. Electron crystallographic studies of the crystals have been made to elucidate the arrangement of the ribosomal subunits on the membrane surface. We have now obtained more extensive and better ordered crystals of the same habit, grown from chick embryo ribosomes, and report here on their native structure preserved by rapid freezing of the crystals in thin aqueous films. The three-dimensional map reveals new details of the protein and ribosomal RNA distribution within the ribosome. Most striking is a region of low density within the large subunit which extends from the subunit interface towards an area on the membrane-facing surface identified by others as the exit site of the nascent protein. This region of low density appears to delineate the path taken by the growing polypeptide through the ribosome to the external surface.