The virulence plasmid-encoded type III secretion system of Shigella flexneri consists of the Mxi-Spa secretion apparatus, secreted proteins IpaA-D and IpgD involved in entry of bacteria into epithelial cells,cytoplasmic chaperones IpgC and IpgE and 15 other secreted proteins of unknown function, including VirA and members of the IpaH family. The activity of the Mxi-Spa apparatus is regulated by external signals, and transcription of virA and IpaH genes is specifically induced in conditions of active secretion. We present genetic evidence that regulation of these genes involves both MxiE, the transcriptional activator of the AraC family encoded by the mxi operon, and IpgC, the chaperone for IpaB and IpaC. We also show that together MxiE and IpgC are sufficient to activatevirA and IpaH 9.8 promoters in Escherichia coli. InS. flexneri, increasing the expression of IpgC led to a concomitant increase in IpaH production in conditions of non-secretion. This suggests that the activity of secretion is sensed by the presence of free IpgC, which acts as a coactivator to allow MxiE to activate transcription at its target promoters.