Ibotenic acid, a constituent of Amanita muscaria, is a potent NMDA receptor agonist. The structure of the ibotenic acid zwitterion monohydrate in the crystalline state has been determined by X-ray crystallography. The crystal structure has two ibotenic acid monohydrate formula units in the asymmetric unit and features an extensive hydrogen bond network. The ibotenic acid zwitterions in the crystalline state are compared to the in vacuo structure calculated by quantum chemical ab initio calculations at the HF/6-31+G* level and the effects of the hydrogen bond network on the structures in the solid state are discussed. The calculated potential energy curve with respect to side-chain orientation displays a single energy minimum. The conformations corresponding to the solid-state conformations are calculated to be ca. 2 kcal/mol higher in energy than the minimum-energy conformation in vacuo.