Ganciclovir (GCV; systematic name: 2-amino-9-{[(1,3-dihydroxypropan-2-yl)oxy]methyl}-6,9-dihydro-1H-purin-6-one), C9H13N5O4, an antiviral drug for treating cytomegalovirus infections, has two known polymorphs (Forms I and II), but only the structure of the metastable Form II has been reported [Kawamura & Hirayama (2009). X-ray Struct. Anal. Online, 25, 51-52]. We describe a successful preparation of GCV Form I and its crystal structure. GCV is an achiral molecule in the sense that its individual conformers, which are generally chiral objects, undergo fast interconversion in the liquid state and cannot be isolated. In the crystalline state, GCV exists as two inversion-related conformers in Form I and as a single chiral conformer in Form II. This situation is similar to that observed for glycine, also an achiral molecule, whose α-polymorph contains two inversion-related conformers, while the γ-polymorph contains a single conformer that is chiral. The hydrogen bonds are exclusively intermolecular in Form I, but both inter- and intramolecular in Form II, which accounts for the different molecular conformations in the two polymorphs.The structure of the thermodynamically stable polymorph I of ganciclovir is reported for the first time and compared with the metastable polymorph II.