The crystal structure of Psilocin, the minor hallucinogenic component of Teonanácatl, the sacred mushroom of Mexico, has been determined. Crystals are monoclinic, space group P21/c, a= 1060(3), b= 853(2), c= 1251(3) pm, β= 91·25(30)°. Data were collected on a linear diffractometer employing graphite monochromatised Mo-Kα radiation. The structure was solved by direct methods and refined by block diagonal least-squares to R 0·047 over 1132 significant data. There are intermolecular hydrogen bonds between the ethylamino-nitrogen and the indole hydroxy-group, and disorder of the proton in this hydrogen bond suggests that the crystal is composed of a statistical mixture of zwitterions and uncharged species. The molecular conformation is different from that found in psilocybin.