The polymorphism of sulfathiazole has been reexamined. The issue of the separate existence of two of the forms, forms III and IV, has now been definitively resolved: both forms exist. Single crystals were used to generate X-ray diffraction patterns, equivalent to those normally obtained from polycrystalline samples, using the ingenious Gandolfi camera. The patterns obtained were distinct and in agreement with the theoretical powder patterns calculated from the reported structures of the two forms, thus confirming the separate existence. Disagreement between the results of previous workers has been traced to two main causes: characterization techniques are not very discriminating for the two forms (III and IV), and the samples examined have invariably been mixtures of III and IV, since the two forms usually crystallize together. Furthermore, crystals of form III exhibit marked preferred orientation, which complicates powder X-ray diffraction (PXRD) analysis. The IR, Raman, and 13C NMR spectra, and PXRD patterns are reported for all four forms. These distinct fingerprints should now enable one to isolate pure samples and to study the properties and the relationships between these forms with full confidence.