Detection limit of PCR was investigated for Renibacterium salmoninarum, the causative agent of bacterial kidney disease, with a logarithmic growth phase. When genomic DNAs from serially diluted bacterial cells were subjected to PCR, the minimum concentration by PCR-detection with 30, 35 and 40 cycles of amplification were all 10⁵ cells/mL, and the sensitivity was not improved by treatments of the bacteria with lysozyme, achromopeptidase and/or SDS. The extraction rate of genomic DNAs from the bacterial cells was approximately 3%. It was confirmed that PCR-detection limit for R. salmoninarum cells mixed with fish kidney tissues was significantly lower than that for the bacteria suspended in the medium. Moreover, there was no significant difference in the detection limit between immunofluorescence antibody technique (IFAT) and PCR. These results suggest that PCR detection should be followed by other methods such as IFAT.