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5'-Methylammonium-5'-deoxyadenosine is a poor subsrate for adenosine desaminase. The nucleoside was crystallized from aqueous methanol as its iodide monohydrate in space group P2(1). The structure was solved from three dimensional X-ray data and refined to R = 4.1%. The structure of the nucleoside is determined by an intramolecular hydrogen bond from N5' to N3 which implies that (1) the edenine heterocycle is in syn position with respect to the sugar moiety; (2) the ribose pucker is C2'-exo, C3'-endo and (3) the conformation about the C4'-C5' bond is trans, gauche. The water of hydration is fourfold disordered and forms a linear hydrophilic region which is surrounded by nucleoside molecules in a zigzag arrangement. The adenine heterocycles are not stacked and in closer than Van der Waals distance with iodine ions suggesting a charge transfer type interaction.