Analytical methods were developed for the determination of a new antibacterial agent, 5, 8-dihydro-5-methoxy-8-oxo-2H-1, 3-dioxolo [4, 5-g] quinoline-7-carboxylic acid (miloxacin) by high performance liquid chromatography (HPLC) and ultraviolet spectrophotometry (UV). Miloxacin was separated from its impurities or degradation compounds by HPLC under the following condition : stainless steel column (2.1 mm i.d.×1 m) packed with strong anionic ion exchanger ; column temperature, 40°; mobile phase, pH 5.0 solution containing 0.01 M citrate buffer and 0.03 M sodium nitrate at the flow rate of 0.7 ml/min ; detector, a UV detector at the wavelength of 254 nm. Miloxacin could be determined with a relative standard deviation of 1% using picolinic acid as internal standard and its impurities were also determined at the same time. Moreover, miloxacin was dissolved in 1% sodium carbonate solution and determined by measuring the absorbance of the solution at the wavelength of 344 nm with a relative standard deviation of 0.3%. The impurities did not interfere with the determination of miloxacin and the results obtained by UV were in good agreement with those obtained by HPLC within experimental error, including some samples partially decomposed.