The structure function of DNA fragments (contour length 54-104 nm) immersed in aqueous salt solutions is determined with small angle neutron scattering. From the limiting behavior at high values of momentum transfer an effective diameter or second moment of the DNA cross section of 1.6 nm is derived. For low values of momentum transfer and/or high ionic strength, the effects of supporting electrolyte, DNA;concentration, and DNA contour length can be rationalized with virial theory including screened electrostatics. With excess monovalent salt concentration exceeding, e.g., 1 M, neutral polymer behavior is recovered.