We compared the in vitro antibacterial activity of DV-7751a against gram-positive and -negative bacteria with those of quinolones currently available. MICs for 90% of the strains tested (MIC90s) against clinical isolates of methicillin-susceptible and -resistant Staphylococcus aureus and Staphylococcus epidermidis were 0.20, 0.39, 0.20, and 0.78 micrograms/ml, respectively. Moreover, MIC50s for DV-7751a against ofloxacin-resistant methicillin-resistant S. aureus were 4-, 8-, 16-, 32-, and 64-fold lower than those for tosufloxacin and sparfloxacin, levofloxacin, ofloxacin and fleroxacin, ciprofloxacin, and lomefloxacin, respectively. DV-7751a inhibited the growth of all strains of Streptococcus pneumoniae, Streptococcus pyogenes, and Peptostreptococcus spp. at 0.39, 0.39, and 0.78 micrograms/ml, respectively, and was 4- to > 16-fold more active against enterococci at the MIC90 level than the other quinolones tested. The activity of DV-7751a against Pseudomonas aeruginosa was roughly comparable to those of levofloxacin and sparfloxacin at the MIC90 level and was two- to fourfold less than that of ciprofloxacin. DV-7751a showed activity comparable to those of levofloxacin and ciprofloxacin against the other glucose-nonfermenting bacteria Haemophilus influenzae, Neisseria gonorrhoeae, and Moraxella catarrhalis (MIC90s of 0.025, 0.20, and 0.10 micrograms/ml, respectively). DV-7751a activity was not affected by medium, inoculum size, or the addition of human serum but was decreased under acidic conditions and in human urine, as were the other quinolones tested. Time-kill curve studies demonstrated the rapid bactericidal action of DV-7751a against S. aureus, S. pneumoniae, Escherichia coli, and P. aeruginosa. The frequency of spontaneous resistance to DV-7751a was less than or equal to those of the reference drugs. DV-7751a inhibited the supercoiling activity of DNA gyrases from S. aureus, E. coli, and P. aeruginosa at concentrations comparable to those of levofloxacin and sparfloxacin.