Trovafloxacin, sparfloxacin, ciprofloxacin and levofloxacin were equally active against Moraxella catarrhalis, Haemophilus influenzae, Legionella pneumophila, Klebsiella pneumoniae, Enterobacter cloacae and Serratia marcescens. Ciprofloxacin was the most active compound against Pseudomonas aeruginosa (MIC90 = 1 mg/L), followed by trovafloxacin (MIC90 = 4 mg/L). Trovafloxacin was twice as active as sparfloxacin against Streptococcus pyogenes (MIC90 = 0.12 mg/L), Streptococcus pneumoniae (MIC90 = 0.12 mg/L) and Staphylococcus aureus (MIC90 = 0.06 mg/L) (except quinolone-resistant, methicillin-resistant S. aureus, for which the MIC90 was 8 mg/L). Trovafloxacin was the most active compound against Enterococcus faecalis: 80% of strains were susceptible to 0.25 mg/L. There was complete cross-resistance between all fluoroquinolones.