The spatial arrangement of nucleosomes in rat liver chromatin has been examined using the electric birefringence technique. All chromatin subunits studied (up to 9 consecutive nucleosomes) contain their full complement of the five histone types associated with about 200 base pairs repeat length DNA. From the relaxation times and the orientation mechanisms, the nucleosome may be assimilated to an oblate ellipsoid of dimensions about 140 x 140 x 70 A, and the DNA superhelical axis is parallel to its shorter axis. The most important result is a sharp transition in the electro-optical properties of subunits when the number of nucleosomes in the chain is greater than 6 : the initial negative birefringence, as for DNA, becomes positive and the relaxation time is multiplied by ten. The hexanucleosome, which presents no birefringence, has an helical symmetrical structure without preferential orientation axis. This structure is approximatively spherical of about 250 A diameter and the chromatin appears as a periodic array of such a structure.