In vitro activity of sitafloxacin (STFX) and various oral antimicrobial agents against bacterial isolates recovered from clinical specimens between January and December 2009, at different healthcare facilities in Japan was evaluated. A total of 1,620 isolates including aerobic and anaerobic organisms was available for the susceptibility testing using the microbroth dilution methods recommended by Clinical Laboratory Standard Institute. The minimum inhibitory concentration of STFX at which 90% of isolates (MIC90) was 0.06 microg/mL for methicillin-susceptible Staphylococcus aureus and was equal to that of garenoxacin (GRNX), 2 times lower than that of moxifloxacin (MFLX), and 8 times lower than that of levofloxacin (LVFX). STFX inhibited the growth of all the isolates of Streptococcus pneumoniae at 0.06 microg/mL or less. The MIC90s of STFX ranged from 0.03 to 0.06 microg/mL and were 1 to 2 times lower than those of GRNX, 2 to 4 times lower than those of MFLX, and 16 to 32 times lower than those of LVFX. Against Streptococcus pyogenes, the MIC90 of STFX was 0.06 microg/mL and was 2 times lower than that of GRNX, 4 times lower than that of MFLX, and 32 times lower than that of LVFX. The MIC90 of STFX was 0.25 microg/mL for Enterococcus faecalis, and was 2 times lower than those of GRNX and MFLX, and 8 times lower than that of LVFX. The MIC90 of STFX for E. coli was 2 microg/mL, and the MIC90s of other 10 species of Enterobacteriaceae which were the lowest values of the quinolones tested ranged from 0.03 to 1 microg/mL. The MIC90 of STFX for Pseudomonas aeruginosa isolates recovered from urinary infections was 8 microg/mL and was 16 times lower than those of GRNX, MFLX and LVFX. The MIC90 of STFX for P aeruginosa isolates recovered from respiratory infections was 2 microg/mL and was 32 times lower than those of GRNX and MFLX, and 16 times lower than that of LVFX. STFX inhibited the growth of all the isolates of Haemophilus influenzae at 0.004 microg/mL or less, and was 2 to 4 times lower than those of GRNX, 8 times lower than those of MFLX, and 4 times lower than those of LVFX. The MIC90 of STFX was 0.008 microg/mL for Moraxella catarrhalis, and was 2 times lower than that of GRNX, 8 times lower than those of MFLX and LVFX. The MIC90s of STFX ranged from 0.015 to 0.12 microg/mL for all the species of anaerobic bacteria and were the lowest values of all the antimicrobial agents tested. In conclusion, the activity of STFX against Gram-positive cocci was comparable or superior to those of GRNX, MFLX and LVFX. STFX showed the most potent activity against Gram-negative bacteria and anaerobic bacteria of all the antimicrobial agents tested in this study.