The S fimbrial adhesin ( sfa) determinant of E. coli comprises nine genes situated on a stretch of 7.9 kilobases (kb) DNA. Here the nucleotide sequence of the genes sfaB and sfaC situated proximal to the main structural gene sfaA is described. Sfa-LacZ fusions show that the two genes are transcribed in opposite directions. The isolation of mutants in the proximal region of the sfa gene cluster, the construction of sfa-phoA gene fusions and subsequent transcomplementation studies indicated that the genes sfaB and sfaC play a role in regulation of the sfa determinant. In addition the nucleotide sequence of the genes sfaD, faE and sfaF situated between the genes sfaA and sfaG responsible for S subunit proteins, were determined. It is suggested that these genes are involved in transport and assembly of fimbrial subunits. Thus the entire genetic organization of the sfa determinant is presented and compared with the gene clusters coding for P fimbriae ( pap), F1C fimbriae ( foc) and type I fimbriae ( fim). The evolutionary relationship of fimbrial adhesin determinants is discussed.