The structure of the title compound has been determined by single-crystal X-ray diffraction. Crystals are orthorhombic with Z= 4 in a unit cell of dimensions: a= 12·952(4), b= 14·710(5), c= 5·943(3)A, spacegroup P212121. The structure was solved by Patterson and Fourier methods and refined by least-squares using 1197 observed three-dimensional diffractometer data to R 0·050. The imidazothiazolium group is almost planar and the angle between the normals to the mean plane and the phenyl group is 31°. The formal double-bond C(7)N(7) does not exist, and electron delocalisation over N(7), C(7), N(4), and S(1), causes this group to be planar. A hydrogen bond between the chlorine atom and N(7) was found.