Seventy-nine isolates of oral streptococci and Aerococcus obtained from blood specimens between December 1997 and January 2000 were tested for high-level gentamicin resistance. Three isolates with minimum inhibitory concentrations (MICs) of ≧1024 μg/ml were selected as potential high-level gentamicin-resistant isolates. The DNA sequence of the structural gene encoding 6′-acetyltransferase-2″-phosphotransferase in the three high-level gentamicin-resistant isolates was investigated according to the method used for Enterococcus faecalis. The results of this gene analysis showed that all the DNA sequence of Aerococcus viridans no. 70, which had the highest MIC (2048 μg/ml), matched that of E. faecalis, whereas no polymerase chain reaction (PCR) products were obtained in the other two isolates that were tested. The mechanism of gentamicin resistance in these two isolates is unknown.