Following oral administration of levofloxacin (LVFX, (S)-(-)-Ofloxacin; formerly designated as DR-3355) at 200 mg per dose 3 times a day for 7 days to 6 healthy male volunteers, degrees of disturbance of the fecal microflora and fecal drug concentrations were examined. The total viable count remained unchanged during the study period due to the minimal change in the number of members of the family Bacteroidaceae, the most predominant organisms. Most of the aerobes including facultative anaerobes were suppressed by LVFX with only a slight increase in yeasts. In particular, the members of the family Enterobacteriaceae were reduced to below the detection limit on and after day 3 through the time of discontinuation of the drug in all subjects but one. Among the obligate anaerobes, peptostreptococci and bifidobacteria decreased or disappeared in some volunteers, but no significant changes were observed in other anaerobes. Neither Clostridium difficile nor its toxin D-1 was detected in any of the volunteers. No side effects attributable to the drug were observed. During administration, LVFX was detected in the feces at high concentrations which correlated well with the decrease of susceptible members of flora as well as to their detection rate.