To determine if susceptibility testing of Streptococcus pneumoniae could detect those isolates that had one of the recognized mechanisms of fluoroquinolone resistance, 101 isolates were selected; the levofloxacin MIC for 28 of these isolates was > or =4 microg/ml. Only isolates with both parC and gyrA mutations or with no recognized resistance mechanisms were reliably identified by using these results. Isolates with only a parC mutation could not be detected reliably using any susceptibility testing method.