Difloxacin is a new quinolone antimicrobial agent with in vitro activity against both Neisseria gonorrhoeae and Chlamydia trachomatis and a long (26-h) half-life. A single oral dose of 200 mg of difloxacin was used to treat 30 men with uncomplicated urethral gonorrhea in an open trial. Of the isolates of N. gonorrhoeae, three produced penicillinase and two were resistant to tetracycline. N. gonorrhoeae was eradicated from all 29 evaluable patients. The geometric mean MIC of difloxacin for 30 pretreatment N. gonorrhoeae isolates was 0.014 (range, ≤0.0039 to 0.03) μg/ml. Four (13.3%) of the 30 subjects with gonococcal urethritis also had C. trachomatis recovered from their pretreatment cultures. Treatment with difloxacin was associated with the eradication of C. trachomatis from all four men. In addition, C. trachomatis was isolated from the posttreatment culture of only one man who had a negative culture before treatment. Nineteen patients (65.5%) reported adverse experiences, and 17 of them (58.6%) developed symptoms suggestive of central nervous system dysfunction. An oral dose of 200 mg of difloxacin is effective treatment for uncomplicated urethral gonorrhea and may also eliminate a coexisting infection with C. trachomatis. Side effects may limit the utility of this agent.