The antibacterial potencies of gemifloxacin (SB-265805) and 13 comparator compounds were determined by broth microdilution against a panel of 645 Gram-positive and 995 Gram-negative organisms collected from various USA sites. Time–kill studies were performed and postantibiotic effect (PAE) was determined for several organisms using trovafloxacin and ciprofloxacin as comparator compounds. Based on MIC90s, gemifloxacin was the most potent compound tested against Gram-positive isolates: Streptococcus pneumoniae (MIC90 0.016 mg/L), Streptococcus agalactiae (0.03 mg/L), Streptococcus pyogenes (0.03 mg/L), viridans streptococci (0.12 mg/L), methicillin-susceptible Staphylococcus aureus (0.03 mg/L), Staphylococcus epidermidis (2 mg/L), Staphylococcus saprophyticus (0.016 mg/L) and Enterococcus faecalis (2 mg/L). Against Gram-negative isolates, the potency of gemifloxacin was equal to that of levofloxacin and ciprofloxacin and generally better than that of ofloxacin, grepafloxacin, trovafloxacin and nalidixic acid. MIC90s for gemifloxacin were: Haemophilus influenzae (≤0.008 mg/L), Moraxella catarrhalis (0.008 mg/L), Escherichia coli (0.016 mg/L), Klebsiella pneumoniae (0.25 mg/L), Klebsiella oxytoca (0.25 mg/L), Enterobacter cloacae (1 mg/L), Enterobacter aerogenes (0.25 mg/L), Proteus spp. (4 mg/L), Serratia spp. (1 mg/L), Citrobacter freundii (2 mg/L), Morganella morganii (0.12 mg/L), Pseudomonas aeruginosa (8 mg/L), Stenotrophomonas maltophilia (4 mg/L) and Acinetobacter spp. (32 mg/L). Gemifloxacin was bactericidal for all organisms studied at 2 and 4 × MIC. The PAE for most strains was in the range 0.7–2.5 h at 2 and 4 × MIC, although longer PAEs were observed with H. influenzae, P. aeruginosa and Proteus vulgaris (>6 h at 4 × MIC) and shorter PAEs with E. faecalis (0.1–0.6 h) and K. pneumoniae (0.1–0.2 h). In conclusion, gemifloxacin is a novel quinolone with a broad spectrum of antimicrobial activity. It has substantially improved potency against Gram-positive organisms, especially streptococci, for which gemifloxacin is generally at least eight- to 16-fold more potent than other quinolones tested. It retains the good Gram-negative activity seen with ciprofloxacin and levofloxacin and shows good bactericidal activity and prolonged PAEs.