Small-angle X-ray scattering patterns were obtained from rabbit reticulocyte ribosomes using a Berreman-type point-focusing single quartz crystal monochromator. These patterns are interpreted to determine the radius of gyration of 105 A to about 2% and an estimate of the molecular weight. No evidence of secondary scattering maxima was observed. An investigation of approximate volume changes between two ionic strengths was made with negative results. The electron density of the sample is approximated by a Gaussian consistent with the data with a cutoff at 170 A, giving in connection with electron microscope data of C. Hall a hydration of 2.7 g water per gram ribosome.