A process for preparing the agricultural herbicide clomazone is provided, the process comprising reacting 4,4-dimethyl-3-isoxazolidinone with 2-chlorobenzyl chloride in an aqueous medium in the presence of a base, in particular an alkali metal hydroxide, carbonate or hydride, for example sodium hydroxide. A method for preparing crystalline clomazone is also disclosed comprising (a) crystallising clomazone from solution in an organic solvent； and (b) isolating the resulting crystals. N,N-dimethylformamide, benzene, toluene, acetonitrile, dichloromethane and hexane are all suitable solvents. Further, there is provided Form I crystalline 2-[(2-chlorophenyl)methyl]-4,4-dimethyl-3-isoxazolidinone (clomazone), wherein the polymorph Form I is characterized by at least one of the following properties: (i) an X-ray powder diffraction pattern having characteristic peaks expressed in degrees 2Ó¨(+/−0.20° Ó¨) at one or more of the following positions: about 10.63, 16.07, 18.08, 19.11, 19.34, 21.20, 24.78 and 28.80； and (ii) an infrared (IR) spectrum having a characteristic peak: at about 2967 and 2870 cm−1.
