To develop a nested..polymerase chain reaction ( nested..PCR ) for detect ion of Renibacterium salmoninarum, the causat ive agent of bacterial kidney disease, 2 pairs of primers, BKDF and BDKR, BKDR2 and BKDF2, were designed for amplif ication of 501 bp and 314 bp DNA fragments of the sequence coding the 57 kDa cell surface protein of R . salmoninarum respectively. No specif ic f ragments were amplif ied when other principal f ish bacterial pathogens were used as templates in PCR and nested..PCR tests. The sensit ivity of PCR was limited to 1. 8 .. 10 3 CFU..mL - 1 when the bacterial DNA was extracted by phenol..chloroform..isopentanol or boiling &f reezing..thawing method. The sensitivity of nested..PCR was 100 times higher than that of PCR test. The mixture of bacteria and cod eggs was detected using nested..PCR and the results showed that the nested..PCR was an effective and reliable method for the detection of R . salmoninarum.