The molecular conformations of the anhydrous and hydrated forms of carnidazole {O-methyl [2-(2-methyl-5-nitro-1H-imidazol- 1-yl)ethyl]carbamothioate } differ essentially in the relative orientations of he nitroimidazole and thiocarbamate moieties. In the anhydrous form, the molecules are connected by a network of hydrogen bonds and show stacking interactions between the nitroimidazole moieties.