The aim of this in vitro study was to investigate the relationship between bacterial infections in carious dentin when detected by two different methods -- polymerase chain reaction (PCR), and a laser fluorescence device, DIAGNOdent. Dentin was removed every 300 micro m in the direction of the pulp chamber in 10 extracted molars with occlusal dentin caries and 3 extracted sound molars. Dentin surfaces were evaluated using DIAGNOdent, and dentinal tissue samples were removed by using a round bur before and after each removal. Bacterial DNA in the dentinal tissues was detected by PCR, using primers based on the nucleotide sequence of a conserved region of the 16S rDNA, and yielded a PCR product of 466 bp. The rates of bacterial detection increased as the DIAGNOdent values increased. In the 10 specimens, the lowest DIAGNOdent value at which bacteria were detected was 15.6; at DIAGNOdent values below 15.6, no bacteria were detected. The results of a receiver operating characteristic (ROC) curve for the DIAGNOdent values showed that the area under the ROC curve was 0.91. This study clarified the relationship between the DIAGNOdent values of dentin caries and the rates of bacterial detection.