A total of 33 Stenotrophomonas maltophilia clinical isolates were tested for their susceptibility to clinafloxacin in comparison with ciprofloxacin, levofloxacin, moxifloxacin, nalidixic acid, norfloxacin, sparfloxacin and trovafloxacin. The MIC50 and MIC90 were as follows: ciprofloxacin 4 and 64 μg/mL; clinafoxacin 0.5 and 4 μg/mL; levofloxacin 2 and 32 μg/mL; moxifloxacin 1 and 8 μg/mL; nalidixic acid 8 and 128 μg/mL; norfloxacin 64 and 256 μg/mL; sparfloxacin 1 and 16 μg/mL; and trovafloxacin 1 and 8 μg/mL. Clinafloxacin was the most active quinolone, with only a 15.1% of strains showing resistance. When the MICs were determined in the presence of 25 μg/ml of reserpine, the MIC90 of trovafloxacin and moxifloxacin did not change, whereas decreased 2-fold for clinafloxacin, levofloxacin, sparfloxacin and nalidixic acid, and 4- and 8-fold for ciprofloxacin and norfloxacin respectively. No clinafloxacin-resistant strains were observed when the MIC was performed in the presence of reserpine. Therefore, clinafloxacin shows the better "in vitro"activity against these 33 strains of S.maltophilia.