4-(3-Carb-oxy-1-ethyl-6-fluoro-4-oxo-1,4-dihydroquinolin-7-yl)piperazin-1-ium 4-carb-oxy-benzoate-benzene-1,4-dicarb-oxy-lic acid (2/1).

In the title compound, C(16)H(19)FN(3)O(3) (+)·C(8)H(5)O(4) (-)·0.5C(8)H(6)O(4), the benzene-1,4-dicarb-oxy-lic acid mol-ecule is located on a centre of symmetry. In the crystal, the mol-ecules and ions are connected by inter-molecular C-H⋯O and O-H⋯O hydrogen bonds and π-π stacking inter-actions [with a centroid-centroid distance of 3.402 (2) Å], generating a three-dimensional supra-molecular structure.