The two polymorphic structures of 3-phenyl-1H-1,3-benzo­diazol-2(3H)-one (IandII) exhibit identical bond distances and angles except for the C—N—C—C torsion angle between the benzimidazolone backbone and the phenyl substituent, which has an effect on the crystal packing and supra­molecular features. The structure ofIcontains a stronger C=O⋯H—N hydrogen-bonding inter­action and a weaker π–π inter­action between adjacent bezimidazolone moieties in comparison toII.