Entry of Shigella flexneri into epithelial cells involves secretory proteins, the Ipa proteins, and their dedicated secretion apparatus, the Mxi-Spa translocon, which is encoded by the mxi and spa operons. We have characterized the mxiG gene that is located at the proximal part of the mxi operon. Inactivation of mxiG abolished lpa secretion, which indicates that MxiG is an essential component of the Mxi-Spa translocon. Immunoblotting analysis of membrane fractions suggests that the 42 kDa MxiG protein is associated with both the inner and outer membranes. Taking advantage of the complementation of the mxiG mutant by a plasmid carrying a wild-type copy of mxiG (which restored Ipa secretion, entry into HeLa cells, and cell-to-cell spread) we mutagenized the mxiG gene carried by the complementing plasmid to replace the RGD motif of MxiG by RAD. This mutation (mxiG*), which had no effect on the stability of the protein, did not affect Ipa secretion in vitro or entry into HeLa cells, but impaired intercellular dissemination. Therefore, MxiG and possibly proteins secreted by the Mxi-Spa translocation are involved not only in entry but also in spread of Shigella between epithelial cells.