The local mobility of DNA and RNA can be described well by a segmented rigid body model in which the phosphates, riboses and bases are treated as independent groups. We have developed a computer program for extracting information about the translational and rotational mobility of these groups from X-ray diffraction data of single crystals of DNA and RNA fragments. We plan to extend our studies from the B DNA helix studied here to A and Z form helices for which diffraction data are available. The mobilities described here may be important in allowing the flexibility necessary for interaction of nucleic acids with proteins, ligands or other nucleic acids, and are crucial to our understanding of the factors governing both structural stability and motion of nucleic acids on a larger scale.