Gatifloxacin, a new advanced-generation, 8-methoxy fluoroquinolone, has shown efficacy against a broad spectrum of microorganisms. In this study, the in vitro activity of gatifloxacin was compared with that of ciprofloxacin, clarithromycin, erythromycin, and rifampin against 214 Legionella spp. Species tested in order of frequency were: L. pneumophila serogroups 1 to 9 (181 strains); L. dumoffii (10 strains); L. micdadei (9 strains); L. longbeachae (7 strains); and other Legionella spp. (7 strains). MICs were determined by a standard dilution procedure using buffered yeast extract agar. Gatifloxacin and rifampin were the most active agents against all strains of Legionella tested. Moreover, against L. pneumophila strains tested, gatifloxacin was found to be more active (highest MIC90 = 0.03 mg/L) than ciprofloxacin (highest MIC90 = 0.06 mg/L) and clarithromycin (highest MIC90 = 0.12 mg/L). L. pneumophila serogroups 1 to 4 and 6 to 9 (MIC90 = 0.016 mg/L) were more susceptible to gatifloxacinthan L. pneumophila serogroup 5 (MIC90 = 0.03 mg/L). The activity of gatifloxacin against L. micdadei was equal to that of ciprofloxacin (MIC90 = 0.016 mg/L) and greater than that of erythromycin (MIC90 = 1.0 mg/L). The activity of gatifloxacin against L. dumoffii and L. longbeachae was equal to that of ciprofloxacin (MIC90 = 0.03 mg/L). The activity of gatifloxacin was similar against isolates obtained from both patients and environmental sources.