Thin vitrified films of 146 base pair double-stranded DNA fragments in semi-dilute solutions are observed by cryo-electron microscopy. Depending on the salt conditions of the buffer and the DNA concentration, the molecules form different characteristic arrangements: they are arranged either locally parallel to each other in random overall orientation or packed perpendicular to the film surface, or else are randomly oriented. These arrangements reflect the weak forces acting between the DNA molecules themselves, as well as between the molecules and the thin film surface.