Gatifloxacin, a new broad-spectrum fluoroquinolone, was given in oral doses of 400 mg once daily for 10 days to 18 healthy volunteers, in order to study the impact on the normal intestinal microflora. Fecal samples were collected prior to administration (Day -4 and Day -1), during the administration period (Day 5 and 10) and after withdrawal of administration (Day 12, 16, 22, 30 and 50). In the aerobic intestinal microflora Escherichia coli strains were eliminated or strongly suppressed during the administration period and the numbers of enterococci decreased significantly while the numbers of staphylococci increased at the same time. In the anaerobic microflora the numbers of clostridia and fusobacteria decreased significantly during the administration period while no other major changes occurred. The microflora was normalized 40 days after the administration of gatifloxacin had stopped. No selection or overgrowth of resistant bacterial strains or yeasts occurred. The ecological impact of gatifloxacin was shown to be selective and similar to that of quinolones like ciprofloxacin, levofloxacin and ofloxacin.