A crystal of 1-butyl-3-methylimidazolium iodide [bmim]I, a prototype ionic liquid, has been prepared and its crystal structure has been determined at the space group P2(1)2(1)2(1). The butyl-chain has gauche and trans configurations, and only one H atom of cation is linked to the iodide anion through a hydrogen bond. The weak hydrogen-bonded network involving the iodide anion is a characteristic feature of [bmim]I, which could be related to the observation that crystallization was accomplished under 233 K.