The title compound, eprosartan mesylate {systematic name: 2-butyl-1-(4-carb-oxy-benz-yl)-5-[(E)-2-carb-oxy-3-(thio-phen-2-yl)prop-1-en-yl]-1H-imidazol-3-ium methane-sulfonate}, C(23)H(25)N(2)O(4)S(+)·CH(3)O(3)S(-), one of the angiotensin II-receptor antagonists, is effective in regulating hypertension, induced or exacerbated by angiotensin II, and in the treatment of congestive heart failure, renal failure and glaucoma. In the eprosartan residue, which appears in this crystal in the cationic imidazolium form, the benzene ring plane is almost orthogonal to that of the imidazole ring, making a dihedral angle of 87.89 (2)°. The thio-phene ring forms dihedral angles of 66.54 (2) and 67.12 (2)° with the benzene and imidazole rings, respectively. The imidazolium NH group and the H atom of the aromatic carboxyl group participate in hydrogen bonds with the the O atoms of the anion, thus forming centrosymmetric aggregates made up of two cations and two anions each. The second carboxyl group further links the above-mentioned aggregates through a conventional centrosymmetric hydrogen-bonding motif into infinite chains along [011].