Wet spun films of DNA show a large anisotropic swelling when hydrated, changing little along the fiber axis but swelling by a factor of 10 or more perpendicular to the axis. High resolution electron microscopy has been performed on films with 1 gram NaCl content per gram DNA (1% Na-DNA), as well as higher salt content Na-DNA and with Li-DNA, before and after swelling. After swelling and vacuum drying, the 1% Na-DNA shows a markedly different structure than the more heavily salted DNA. The 1% Na-DNA consists mostly of amorphous regions and plate-like lamellae with only a small percentage of fibrous material remaining. However, the stability of these preparations presumably depends on these fibrous regions which act as tie points for the amorphous matrix. The more heavily salted Na-DNA and Li-DNA do not contain such regions on swelling, which may contribute to the solubility of these materials. (Copyright (c) 1986 John Wiley and Sons, Inc.)