A total of 106 clinical isolates of Staphylococcus aureus were classified into nine pattern types correlated with gyrA mutations. In 62 strains, mutations were found at a single codon (84, 85, 86 and 88), while 22 strains showed double mutations either at codons 84 and 85 or at codons 84 and 88. The double mutations led to the highest levels of ofloxacin resistance (MIC, > or = 128 microg/ml). All isolates with a single Ser-84--> Leu change had an ofloxacin MIC of 8-128 microg/ml, whereas others showed an MIC range of 8-16 microg/ml. Twenty-two wild type strains and one strain with a single mutation at codon 86 (silent mutation) were ofloxacin-susceptible. Thus, gyrA mutations seem to play a definite role in the high-level of resistance to ofloxacin.