The invention relates to crystalline S-(-)-9-fluoro-6,7-dihydro-8-(4-hydroxypiperidin-1-yl)-5-methyl-1-oxo-1H,- 5H-benzo[i,j]quinolizine-2-carboxylic acid L-arginine salt tetrahydrate, a process for its preparation and pharmaceutical formulations incorporating it as the active ingredient for use in treating microbial infections.