Image reconstruction to 22 A resolution of the histone octamer (H3)2(H4)2(H2A)2(H2B)2 shows it to have a 2-fold axis of symmetry, and the overall shape of the left-handed helical spool on which to wind about two turns of a flat superhelix of DNA in the nucleosome. From this structure and the results of various cross-linking studies, we have deduced the arrangement of the individual histones. We propose that the (H3)2(H4)2 tetramer forms a dislocated disk which defines the central turn of DNA, while the two H2A-H2B dimers lie one on each face, each associated with about one half a turn.