The susceptibility of 492 Enterobacteriaceae, 227 other gram-negative bacteria, 448 gram-positive bacteria and 108 anaerobic organisms was determined by the agar dilution method against trovafloxacin and other antibiotics. Trovafloxacin was highly active against most of the Enterobacteriaceae including Enterobacter spp. and Citrobacter spp. [minimum inhibitory concentration (MIC)90 <1 mg/l], Acinetobacter spp. and Pseudomonas aeruginosa (MIC90 = 0.25 and 2 mg/l, respectively). The antimicrobial activity was extended to the gram-positive bacteria including streptococci, Streptococcus pneumoniae, coagulase-negative staphylococci and methicillin-sensitive Staphylococcus aureus with MIC90 <1 mg/l. Enterococci and methicillin-resistant S. aureus were inhibited (MIC90 = 2 mg/l; sparfloxacin and ciprofloxacin were 16 and 64 mg/l, respectively). Almost all anaerobic organisms were inhibited by trovafloxacin (MIC90 = 1 mg/l).