X-ray diffraction patterns have been obtained from partially oriented samples of 300A chromatin filaments. The chromatin was prepared by methods that preserve its structure, and conditions were found in which the 300A filaments spontaneously form ordered aggregates, so that it was not necessary to pull fibers. The diffraction patterns show a meridional band at 110A, and equatorial bands at 340, 57, 37, and 27A. These patterns, together with patterns calculated from the known 7A electron density map of the nucleosome core particle, imply side-to-side packing of nucleosomes in the direction of the 300A filament, and radial packing around it. These results are consistent with the "solenoid" model of Finch and Klug, and are inconsistent with many other proposed models.