Scanning force microscopy has been used to study the structure of chromatin fibers at low salt concentrations. Chicken erythrocyte chromatin fibers in low ionic strength buffer solutions were deposited on mica and imaged in ambient conditions with a tapping mode scanning force microscope. Individual nucleosomes can be clearly discerned in the images of the fibers. Native chromatin fibers show an asymmetrical, 3D structure of sinuous fiber trajectory with irregularly positioned nucleosomes. Fibers depleted of linker histones H1 and H5 have a completely extended 'beads-on-a-string' structure, with linker DNA visible between single nucleosomes. Molecular modeling of the fiber architecture and computer simulation of the imaging process provided more evidence on the observed organization of chromatin at low salt conditions. These results have implications on mechanisms of transcription control and chromatin compaction.