Objective To investigate the transcriptional regulation of quorum sensing (QS) regulators AphA, ToxR, and QsvR on the expression of the phosphodiesterase (GepA) gene gepA in Vibrio parahaemolyticus. Methods Total RNAs were extracted from the wild type (WT) and the mutant strains of aphA, toxR, and qsvR. Quantitative real-time PCR (qPCR) was carried out to calculate the transcriptional variation of gepA between WT and mutant strains. The regulatory DNA region of gepA was cloned into the upstream region of promoterless luxCDABEreporter gene in the pBBRlux plasmid. The recombinant plasmid was respectively transferred into the WT and mutant strains. Luminescence assay was used to test the regulatory effect of QS regulators on the expression of gepA. The primer extension assay was employed to detect the transcription start site and the promoter activity of gepA. The effects of QS regulators on gepA were evaluated based on the abundance of primer extension products. The regulatory DNA region of gepA was cloned into the upstream region of lacZ in the pHRP309 plasmid. The LacZ recombinant plasmid was transformed into EC100 λpir harboring pBAD33 or PBAD33-qsvR. Two-plasmid LacZ reporter assay was conducted to investigate the regulatory effects of QS regulators on the transcription of gepA in EC100 λpir. The regulatory DNA region of gepA was amplified by PCR, and the His recombinant proteins of QS regulators were purified. The electrophoretic mobility shift assay (EMSA) was performed to investigate whether QS regulators directly regulated the expression of gepA. Results At low cell density, the qPCR results showed that expression of gepA in ΔaphA and ΔtoxR were significantly lower than that in WT, indicating that AphA and ToxR activated the transcription of gepA. The luminescence assay showed that the transcriptional activity of the promoter region of gepA in ΔaphA and ΔtoxR was significantly lower than that in WT, further indicating that AphA and ToxR promoted the transcription of gepA. The primer extension assay detected that the transcription start site of gepA was located at the A nucleotide 30 bp upstream of the start codon ATG, and its transcriptional activity was activated by AphA. The EMSA result indicated that His-AphA and His-ToxR were unable to bind the promoter DNA region of gepA. At high cell density, both the qPCR and primer extension assay indicated that QsvR inhibited the transcription of gepA. The EMSA result demonstrated that His-QsvR directly bound to the promoter DNA region of gepA. Two-plasmid lacZ reporter assay demonstrated that QsvR inhibited the transcriptional activity of the promoter region of gepA in EC100 λpir. Conclusion AphA and ToxR indirectly activate while QsvR directly inhibits the transcription of gepA. Therefore, the transcription level of gepA is higher at low cell density and significantly decreases at high cell density.