The ability of Listeria monocytogenes to invade non-phagocytic cells is important for development of a systemic listeriosis infection. The authors previously reported that a L. monocytogenes DsigB strain is defective in invasion into human intestinal epithelial cells, in part, due to decreased expression of a major invasion gene, inlA. To characterize additional invasion mechanisms under the control of s B , mutants were generated carrying combinations of in-frame deletions in inlA, inlB and sigB. Quantitative assessment of bacterial invasion into the human enterocyte Caco-2 and hepatocyte HepG-2 cell lines demonstrated that s B contributes to both InlA and InlB-mediated invasion of L. monocytogenes. Previous identification of the s B -dependent P2 prfA promoter upstream of the major virulence gene regulator, positive regulatory factor A (PrfA), suggested that the contributions of s B to expression of various virulence genes, including inlA, could be at least partially mediated through PrfA. To test this hypothesis, relative invasion capabilities of DsigB and DprfA strains were compared. Exponential-phase cells of the DsigB and DprfA strains were similarly defective at invasion; however, stationary-phase DsigB cells were significantly less invasive than stationary-phase DprfA cells, suggesting that the contributions of s B to invasion extend beyond those mediated through PrfA in stationary-phase L. monocytogenes. TaqMan quantitative reverse-transcriptase PCRs further demonstrated that expression of inlA and inlB was greatly increased in a s B -dependent manner in stationary-phase L. monocytogenes. Together, results from this study provide strong biological evidence of a critical role for s B in L. monocytogenes invasion into non-phagocytic cells, primarily mediated through control of inlA and inlB expression.