We evaluated the in vitro activities of ofloxacin, levofloxacin, grepafloxacin, trovafloxacin and ciprofloxacin against Clostridium difficile. The MIC(90) was 128 mg/L for ofloxacin and levofloxacin, 64 mg/L for ciprofloxacin, 16 mg/L for grepafloxacin and 8 mg/L for trovafloxacin. Thirty per cent of isolates were resistant to trovafloxacin, and rates of resistance to ofloxacin, levofloxacin, grepafloxacin and ciprofloxacin were considerably higher. None of the antimicrobials studied would be a reliable therapeutic option against C. difficile. Whether some of the new fluoroquinolones can induce C. difficile-associated diarrhoea remains to be answered.