The in vitro activity of a new fluoroquinolone, rufloxacin, was determined against both clinical isolates and standard strains of Legionella pneumophila, Chlamydia trachomatis, and Listeria monocytogenes and compared to that of ciprofloxacin, ofloxacin, and erythromycin. Among the antibacterials tested erythromycin was the most active. Rufloxacin inhibited the growth of L. pneumophila at concentrations in the range of 0.06-0.25 mg/l, that of C. trachomatis at concentrations in the range of 2-4, and that of L. monocytogenes at concentrations in the range of 1-2 mg/l. Rufloxacin demonstrated approximately the same activity as ciprofloxacin against C. trachomatis, but twice as less potency as ofloxacin and erythromycin against L. pneumophila, L. monocytogenes, and C. trachomatis, and twice as less potency as ciprofloxacin against L. pneumophila.