The structures of light degradation products of (-)-(S)-10-(1-aminocyclopropyl)-9-fluoro-3-methyl-7-oxo-2,3-dih ydr o-7H- pyrido[1,2,3-de][1,4]benzoxazine-6-carboxylic acid (T-3761) in aqueous solution and in the solid state were investigated. In aqueous solution, ten products were detected using HPLC and in the solid state, one main product was detected. All these light degradation products were isolated and their structures were elucidated. It was revealed that in almost all light degradation products only the 1-aminocyclopropyl side chain at the 10-position of T-3761 is changed to various structures by light oxidation. Surprisingly we found a unique light degradation product, in which carboxylic acid at the 6-position of T-3761 is changed to the hydroxy moiety.