6-Benzyl-3-methyl-1-(β-D-ribofuranosyl)-7H-pyrazolo[4,3-d]pyrimidin-7-one orthorhombic, space group P212121, with a 5.442(1), b 13.387(2), and c 23.759(4) Å; Z = 4 for d = 1.43. The final R = 0.039 for 1154 reflections. The conformational parameters of the compd. are: a C(3')-endo (3E/3T2) ribose conformation, a mid-anti glycosidic bond rotation [χ = 66.4(4)°] and a gauche-trans conformation around the ribose C(4')-C(5') bond. The H bonding in the crystal is analyzed in terms of graph formalism and correspond (according to the formalism of Kuleshova and Zorky 1980) to the F66(3,4,6) or F68(2,3,4,6) graphs according to whether the bifurcated H bonds are single or double bonds. At. coordinates are given. [on SciFinder(R)]