A single crystal of 1-butyl-3-methylimidazolium chloride [bmim]Cl, a prototype ionic liquid, has successfully been prepared and an X-ray crystallographic analysis has been performed. It reveals the presence of a hydrogen bonding network involving the chloride anion and the ring as well as the n-butyl hydrogen atoms, a strong hydrophobic interaction between the two n-butyl groups of the [bmim] cations, and a unique corrugated sheets structure of the imidazolium rings. Considering the similarity of the Raman spectra of crystalline and liquid states, we suspect that the three dimensional structure determined in the present study also exist, at least partially, in the ionic liquid state.