The genes responsible for the expression of type 1 fimbriae, produced by the majority of E. coli strains, have been cloned from an E. coli K12 strain. The "passenger" DNA from an initial cosmid clone was reduced in size and subcloned in pACYC184 and pBR322 vectors. A DNA fragment of around 8 kbp was found to be required for the biosynthesis of type 1 fimbriae. This was further studied by transposon-mediated insertional inactivation and by BAL31-mediated deletions. Four genes, designated fimA, B, C, and D were found to be involved in the synthesis of the fimbriae. They encoded proteins that in their processed form appeared with apparent molecular weights of 16.5 kd, 23 kd, 26 kd, and 89 kd, the 16.6 kd polypeptide being the fimbrial subunit. The order to the genes was found to be: fimB, fimA, fimC, and fimD, organized in three transcriptional units.