Abstract The arrangement of the nucleosome-containing DNA fibers of chromosomes in mitotic HeLa cells was studied by thin sectioning and electron microscopy. Thin sections of mitotic cells that were fixed in growth medium with glutaraldehyde before electron microscopy show a uniform appearance in which the substructure of the chromosomes is not clearly defined. The general distribution of the fibers is made apparent in mitotic cells that are hypotonically swollen while maintaining the chromosomes in a condensed state with divalent cations. The fundamental organization of the chromosomal fibers (of around 250 Å) is seen to be a radial distribution of fibers from the centers of the chromatid arms, probably arranged as loops. It remains possible that a certain fraction of the fibers are longitudinally oriented near the centers of the chromatids.