The crystal structure of sulfathiazole [4-amino-N-(2,3-dihydro-2-thiazolylidene)benzenesulfonamide, C 9 H 9 -N 3 O 2 S] polymorphic form V has been determined from high-resolution synchrotron X-ray powder diffraction data. The structure is monoclinic, space group P2 1 /n, Z = 8, with two molecules in the asymmetric unit. The unit-cell dimensions are a = 14.3296 (3), b = 15.2733 (2), c = 10.4428 (2) A Ê and = 91.052 (1) with cell volume V = 2285.13 (8) A Ê 3 . The structure has been solved by direct methods without recourse to any computational modelling techniques for generating possible structures. The unre®ned structure obtained from direct methods gave an R wp value of 36.5%. Re®nement of atomic and displacement parameters yielded a ®nal R wp of 12.54% (R p = 9.37%). The conformations of the two molecules in the asymmetric unit are nearly identical and very similar to that found in other forms of sulfathiazole. The molecular packing is characterized by molecular sheets lying perpendicular to the a axis. Each sheet is two molecules thick, being integrated by hydrogen bonding. With 16 non-H atoms in the molecule and two molecules in the asymmetric unit, this structure represents a further advance in terms of the complexity of an organic structure solved from X-ray powder diffraction data.