In the previous paper, the production, isolation, and chemical and biological properties of oxetanocin have been reported. The crude material of oxetanocin was purified by a Diaion HP-20 column chromatography and crystallization from water as colorless needles. X-Ray study was first carried out for this free oxetanocin crystal using a Philips PW 1100 diffractometer, with crystal data and the course of structure determination summarized in Table 1(a). Of the 1400 theoretically possible reflections within the 2θ range of 6° through 156°, 1230 reflections were measured above the 2σ(I) level. Intensities were corrected for Lorentz and polarization factors but not for absorption, and hydrogen atoms were located on a difference electron-density map. After elucidating the structure of the free base, the absolute configuration was determined by the anomalous dispersion method using its hydrobromide, which was prepared by dissolving 15.7 mg of oxetanocin in 0.62 ml of 1 M HBr and crystallizing from a 10% (w/v) isopropyl alcohol-water mixed solution to give well-developed plate crystals. The X-ray diffraction work for the hydrobromide was performed analogously to the free base, with crystal data summarized in Table 1(b). 329 Friedel pairs were measured, and from the difference in observed structure factors between hkl and hkl, the absolute configuration at C2' was found to be R, identical to natural nucleosides with a β-D-ribose moiety. The molecular structures of the free base and hydrobromide salt were drawn using the PLUTO program. Selected torsional angles are listed in Table 2. The oxetane ring is puckered along the line through O1' and C3'. The adenine rings are oriented almost perpendicularly to b and stacked along the diad screw axis parallel to b, with the distance between stacked base planes approximately half the axial length of b.