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A simple and rapid method has been developed for the determination of miloxacin (MLX) and its principal metabolite, M-1 {5,8-dihydro-8-oxo-2H-1,3-dioxolo(4,5-g)quinoline-7-carboxylic acid), in fish by high-performance liquid chromatography (HPLC) with fluorescence detection. MLX is extremely unstable in aqueous solution toward light, and is rapidly converted to its demethoxy derivative, M-1. The drugs were extracted from fish with 0.2% metaphosphoric acid-acetonitrile (6: 4, v/v), and the extracts were cleaned up on an Oasis HLB cartridge (60 mg). HPLC separation was performed on a Wakosil II 5C18-RS column (150 X 4.6 rum i.d.) using 25 mM phosphate buffer (pH 2.5)-acetonitrile (70 :30, v/v) as the mobile phase at a flow rate of 0.6 ml/min. The calibration graphs for both compounds were rectilinear from 0.02 to 1 mu g/ml. The recoveries of the compounds from fish fortified at a level 0.1 mu g/g were 80.0 similar to 96.0%, with high precision. The limits of determination of the compounds in fish were 0.01 mu g/g.