The genes responsible for the formation of the F7 (2) fimbriae of the uropathogenic E. coli strain AD110 (O6:K2:H1: F7 ) have been cloned on the recombinant plasmid pPIL110 -35 (Van Die et al. 1983). The F7 (2) fimbriae, like the F7 (1) fimbriae of AD110 , are responsible for mannose resistant haemagglutination ( MRHA ). The molecular organisation of the genes of pPIL110 -35 involved in the expression of MRHA was studied by: (a) analysis of transposon gamma delta and Tn5 insertion mutants. Mutations that cause an MRHA -deficient phenotype were located in discrete groups within an 11.5 kb restriction fragment of pPIL110 -35, separated by insertion mutations that do not inactivate MRHA . (b) complementation experiments. Restriction fragments of pPIL110 -35 subcloned in the vector pBR322 were tested for their ability to complement transposon insertion mutations in the corresponding regions of pPIL110 -35. Five complementation groups were distinguished. Five genes (designated A-E) involved in the expression of MRHA can be distinguished by these results. The products of these genes were analysed in minicells. The results indicate that gene B codes for a 75 K dalton protein, gene C for a 23 K dalton protein and gene E for a 36 K dalton protein. No product of gene D was observed. Gene A probably codes for the 17 K dalton subunit polypeptide of the F7 (2) fimbriae, as will be discussed.