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Image reconstruction to 22 Å 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 a 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.