Salmonella typhimurium exhibits a distinct tropism for mouse enterocytes that is linked to their expression of type 1 fimbriae. The distinct binding traits of Salmonella type 1 fimbriae is also reflected in their binding to selected mannosylated proteins and in their ability to promote secondary bacterial aggregation on enterocyte surfaces. The determinant of binding in Salmonella type 1 fimbriae is a 35-kDa structurally distinct fimbrial subunit, FimHs, because inactivation of fimHs abolished binding activity in the resulting mutant without any apparent effect on fimbrial expression. Surprisingly, when expressed in the absence of other fimbrial components and as a translational fusion protein with MalE, FimHs failed to demonstrate any specific binding tropism and bound equally to all cells and mannosylated proteins tested. To determine if the binding specificity of Salmonella type 1 fimbriae was determined by the fimbrial shaft that is intimately associated with FimHs, we replaced the amino-terminal half of FimHs with the corresponding sequence from Escherichia coli FimH (FimHE) that contains the receptor binding domain of FimHE. The resulting hybrid fimbriae bearing FimHEs on a Salmonella fimbrial shaft exhibited binding traits that resembled that of Salmonella rather than E. coli fimbriae. Apparently, the quaternary constraints imposed by the fimbrial shaft on the adhesin determine the distinct binding traits of S. typhimurium type 1 fimbriae.