SummaryOur laboratories have independently identified a gene inSalmonella choleraesuisandSalmonella typhimuriumthat is necessary for efficient adherence and entry of these organisms into cultured epithelial cells. Introduction of a mutated gene into severalSalmonellastrains belonging to different serotypes rendered these organisms deficient for adherence and invasion of cultured cells. This effect was most pronounced in the host‐adapted serotypesSalmonella gallinarum, S. choleraesuis, andSalmonella typhi.The nucleotide sequence of this gene, which we have termedinvH, encodes a predicted 147‐amino‐acid polypeptide containing a signal sequence. The InvH predicted polypeptide is highly conserved inS. typhimuriumandS. choleraesuis, differing at only three residues. TheinvHgene was expressed inEscherichia coliusing a T7 RNA polymerase expression system and a polypeptide of ∼16000 molecular weight was observed, in agreement with the predicted size of its gene product. Upon fractionation, the expressed polypeptide was localized in the bacterial membrane fraction. Southern and colony hybridization analyses indicated that theinvHgene is present in allSalmonellastrains tested (91 strains belonging to 37 serotypes) with the exception of strains ofSalmonella arizonae.No homologous sequences were detected inYersinia, Shigella, Proteus, and several strains of enteroinvasive and enteropathogenicE. coli.Downstream from theS. choleraesuis(but notS. typhimurium) invHgene, a region with extensive homology to the insertion sequence IS3was d