The key genes, SpnI and SpnR, involved in AI-1-quorum sensing system of Serratia marcescens strain H30 were cloned and localized using specific primers (5′-CTTGAACTGTTTGACGTCAGC-3′ and 5′-AGCGGCCAGGTAATAACTGA-3′, 5′-GCCTTCAATGAAAATCAGACC-3′ and 5′-TGTCGCTGTGATAAGCTCCA-3′) designed according to the nucleic acid sequences published at NCBI (accession no. AB234869). The PCR result demonstrated that the genes SpnI and SpnR were located on the bacterial chromosome and plasmid, respectively. This was also confirmed by Southern blotting using an internal fragment (379 bp) from SpnR gene as a probe. These results imply a new type quorum sensing regulation system that had never been reported previously.