Crystals and other regular arrangements of nucleosome cores have been obtained and analyzed in the electron microscope. Two types of regular structures have been studied in detail, the nucleosome arcs and cylinders. The latter are composed of concentric cylindrical layers of intertwined right-handed helices of nucleosome cores. These studies lead to the following conclusions and concepts. The overall structure of the nucleosome core is a short, wedge-shaped cylinder measuring about 110 by 110 by 60 angstroms. Nucleosome cores interact primarily between top and bottom planes. Nucleosome cores exhibit large conformational variability. A pivot allowing two degrees of rotational freedom is postulated in the region of the 70th base pair to account for this property of the nucleosome.