Vibrio vulnificus MARTX encoded by rtxA, an open reading frame of the rtxHCA operon, is essential for virulence in vitro and in mice. In this study, a primer extension analysis revealed that transcription of the rtxHCA operon begins at a single site, and is under the direction of a single promoter, P( rtxHCA ). P( rtxHCA ) activity appeared at the beginning of growth and reached a maximum in mid-exponential phase. P( rtxHCA ) activity was induced by exposure to INT-407 cells, and the membrane fraction of INT-407 cells was the most effective for the induction.