Six different crystalline modifications of acemetacin, a carboxymethyl ester of indomethacin, were obtained and characterized through the studies on X-ray diffractometry, IR spectroscopy, differential scanning calorimetry, thermogravimetry and solubility measurement. The polymorphs obtained were classified into four anhydrous forms (designated as α, β, γ and δ forms) and two monohydrous forms (designated as A and D forms). Among the anhydrous forms, α, β and γ forms gave a similar melting point of 151-153°C and were not transformed to one another. On the other hand, the δ form melting at 132-134°C was transformed slowly to α form at this temperature. As for the hydrous forms, A form as well as D form were changed to the corresponding anhydrous forms, namely α form and δ form, when heated up to around 80°C. The molecular structure of each crystalline form was investigated by comparing the wave number of carbonyl groups on infrared spectra. The stability of six different crystalline forms when exposed to ultraviolet light as well as elevated temperature (130°C) was investigated and compared with that of indomethacin (stable form, mp 160-162°C).