Quorum sensing is known to be related to regulation of virulence factors in some pathogens. We report transcriptional analysis of cdt genes (cdtA, B, and C ) encoding cytolethal distending toxin (CDT) in Campylobacter jejuni and demonstrate that a luxS mutation affects the cdt transcription in C. jejuni. The reverse transcriptase-polymerase chain reaction (RT-PCR) showed that cdtA, cdtB, and cdtC genes constitute a polycistronic operon in C. jejuni. A putative transcriptional start site was found at 81-bp upstream from the start codon of cdtA by the primer extension assay. A decrease in cdt transcription was observed in the luxS null mutant, which is defective in AI-2 production. The level of cdt transcription in the luxS mutant was approximately 61% of that in the wild-type. Flow cytometric analysis showed that the number of HeLa cells arrested in the G (2) /M phase, a common biological feature of CDT activity, was reduced by treatment with the cell-free supernatant of the luxS mutant as compared to that of the wild-type. Our observations suggest that the function of luxS is related to the regulation of cdt in C. jejuni.