OBJECTIVE: To investigate the in vitro potency and spectrum of activity of gatifloxacin and five comparator fluoroquinolones tested against over 23 000 clinical isolates from diverse geographic and clinical sources in the Americas. METHODS: Gram-negative, Gram-positive and fastidious bacterial isolates were tested against gatifloxacin, ciprofloxacin, levofloxacin, ofloxacin, sparfloxacin and trovafloxacin using broth microdilution methods recommended by the National Committee for Clinical Laboratory Standards (NCCLS). RESULTS: Gatifloxacin demonstrated a potency and spectrum very similar to those of other new fluoroquinolones such as levofloxacin, sparfloxacin, and trovafloxacin. Gatifloxacin was particularly active against the Enterobacteriaceae (94.8% susceptible at </=2 mg/L), Acinetobacter spp. (77.2%), Stenotrophomonas maltophilia (75.1%), Streptococcus pneumoniae (99.8%), other Streptococcus spp. (>/=98.9%), and various Staphylococcus spp. (79.2-100.0%). Trovafloxacin was the most similar comparison drug overall. CONCLUSIONS: These results indicate a potential therapeutic role for gatifloxacin that would widen the potency or spectrum of fluoroquinolones, particularly against Gram-positive species, when considering its favorable bioavailability.