The in vitro and in vivo antibacterial activities of a new fluoroquinolone, DWP20364(1-cyclopropyl-5-amino-6,8-difluoro-7-(2,7-diazabicyclo[3.3.0]oto-4-ene-7-yl)-1 ,4-di-hydro-4-oxoquinoline-3-carboxylic acid) were evaluated in comparison with those of ciprofloxacin(CPFX), sparfloxacin(SPFX) and ofloxacin(OFLX). DWP20364 was more potent than CPFX and OFLX against Staphylococcus spp., Streptococcus spp. and Enterococcus faecium MD8b and it was similarly or slightly less active than CPFX against Escherichia spp. and Pseudomonas spp.. For MRSA and OFLX resistant strains (Staphylococcus spp.(14),Enterococcus spp.(4), Acinetobacter spp.(2), Pseudomonas spp.(9), Klebsiella spp.(2) and Serratia spp.(6)),DWP20364(MICs for 90% of strains,0.025 and 12.5${mu}$g/ml, respectively) was 4 to 32 folds more potent than SPFX and CPFX. The activity of DWP20364 decreased moderately in the presence of 5mM $Mg^{2+}$. However, various pHs and the concentrations of various serum had no effect on the activity of DWP20364. DWP20364 possessed a bacteriocidal effect at the 1MIC against gram positive and gram negative strains. The protective effect of DWP20364 against systemic infections in mice caused by S. aureus Smith or S. aureus L2379 was superior to that of CPFX and SPFX but it was less active than that of CPFX against infection by P. aeruginosa E-2.