We determined in vitro activities of gatifloxacin and seven other drugs against 100 isolates of Stenotrophomonas maltophilia using the agar gradient diffusion (Etest) method. Percentages of susceptible isolates were as follows: trimethoprim-sulfamethoxazole, 90%; gatifloxacin, 71%; levofloxacin, 57%; ticarcillin-clavulanic acid, 54%; ceftazidime, 49%; ciprofloxacin, 29%; cefepime, 21%; and piperacillin-tazobactam, 20%. Time-kill studies of three isolates indicated that gatifloxacin was bactericidal at times as early as 3 h of incubation when tested at concentrations equivalent to twice the MIC (two isolates) and 4 times the MIC (one isolate).