BACTERIAL adhesion pill 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(1). P-pili are 68 Angstrom in diameter and similar to 1 mu m long(2), and are composed of similar to 1,000 copies of the principal structural protein, PapA(3). They are attached to the outer membrane by a minor structural protein, PapH(4) and are terminated by an similar to 20 Angstrom diameter fibrillus composed of PapK, PapE and PapF, which presents the host-binding adhesin PapG(5-7). The amino-acid sequences of PapA(3,8), PapE(9), and PapF(9) are similar, with highly conserved C-termini being responsible for binding to PapD(10-12), the periplasmic chaperone. Our three-dimensional reconstruction indicates that pill 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.