A method has been developed that automatically fits double-helical regions into the electron density of nucleic acid structures. Rigid fragments consisting of two Watson-Crick base pairs and three pairs of phosphate groups in the A-type or B-type conformation are positioned into the electron density by phased rotation and translation functions. The position and orientation of the localized double-helical fragments are determined by phased refinement. The method has been tested by building double-helical regions of nine RNA structures of variable crystallographic resolution and polynucleotide length and is available for free use.