We have grown single crystals of long DNA molecules, 54 and 65 base pairs in length which encompass the binding site of the Xenopus protein transcription factor IIIA (TFIIIA). These molecules are considerably longer than those previously crystallised. X-ray diffraction shows that the crystals are ordered to a resolution of 9 A. The DNA molecules are arranged side to side in layers which are orientated at 120 degrees with respect to adjacent layers. The phosphate backbones of molecules in adjacent layers are interdigitated in both the major and minor grooves. The intensity distribution indicates that the average structure is B-like, although CD spectra in solution are more consistent with the structure, intermediate between A- and B-, found in crystals of a nonamer fragment of the binding site. The B-character in our crystals may therefore be impressed by the tight packing between layers.