The crystal structure of 2-Methyl-3-phenyl-4-bromoisoxazolin-5-one has been determined from three-dimensional X-ray diffraction data. The crystals have the composition C₁₀H₈O₂NBr and are monoclinic, space group P 21/n. There are 4 molecules in the unit cell, which has dimensions: a= 7-551 ± 0.001; b= 18.310 ± 0.009; c = 7.626 ± 0.005 Å; ß = 100.85 ± 0.07°. The position of the bromine atom was found by a three-dimensional Patterson function. With the aid of three subsequent three-dimensional Fourieir summations all the remaining non-hydrogen atoms were located. An isotropic least-squares refinement was carried out for two cycles. The final R value is 0.147. The isoxazoline ring makes an angle of about 50° with the phenyl group; both rings are essentially planar. The adjacent molecules are held together only by van der Waals forces. All intermolecular distances of less than 4 Å were calculated. The shortest distance is 3.12 Å.