The in vitro antibacterial activity of temafloxacin hydrochloride (TA-167 or A-62254) was evaluated against a wide variety of clinical isolates and compared with those of other fluoroquinolones. The potency (MIC90) of the compound against gram-positive aerobic bacteria was higher or comparable to those of ciprofloxacin, ofloxacin, and norfloxacin. Against most gram-negative enteric bacteria and Pseudomonas species, temafloxacin was less active than ciprofloxacin, but was generally as active as ofloxacin and norfloxacin, except against Proteus species and Morganella morganii. Against obligate anaerobes, it was more active than the reference quinolones. Temafloxacin was bactericidal for one strain each of Staphylococcus aureus. Escherichia coli, and Pseudomonas aeruginosa. The compound inhibited E. coli DNA gyrase activity at a low concentration.