Bacterial adhesion pili are designed to bind specifically and maintain attachment of bacteria to target cells. Uropathogenic P-pili are sufficiently mechanically resilient to resist the cleansing action of urine flow that removes most other bacteria. P-pili are 68 A in diameter and approximately 1 micron long, and are composed of approximately 1,000 copies of the principal structural protein, PapA. They are attached to the outer membrane by a minor structural protein, PapH and are terminated by an approximately 20 A diameter fibrillus composed of PapK, PapE and PapF, which presents the host-binding adhesin PapG. The amino-acid sequences of PapA, PapE, and PapF are similar, with highly conserved C-termini being responsible for binding to PapD, the periplasmic chaperone. Our three-dimensional reconstruction indicates that pili are formed by the tight winding of a much thinner structure. A structural transition allows the pilus to unravel without depolymerizing, producing a thin, extended structure five times the length of the original pilus.