Tosufloxacin was highly active in vitro against most isolates of Enterobacteriaceae. It was slightly less active than ciprofloxacin for most species but more active than temafloxacin. It was the most active agent against Acinetobacter spp., Xanthomonas maltophilia and some Pseudomonas spp. but was slightly less active than ciprofloxacin against the fluorescent pseudomonads. Tosufloxacin was highly active against strains of Haemophilus influenzae, Moraxella catarrhalis, Neisseria gonorrhoeae and Campylobacter coliljejuni. It was the most active of the fluoroquinolones tested against staphylococci (MICs 0·016–0·25 mg/L), streptococci (MICs 006-1 mg/L) and enterococci (MICs 0·12–4 mg/L). Tosufloxacin was also the most active quino-lone tested against anaerobic bacteria and, with the exception of a few isolates of Bacteroides spp. and some clostridia, all anaerobes had tosufloxacin MICs > 1 mg/L.