Human adenoviruses are responsible for respiratory, gastroenteric and ocular infections and can serve as gene therapy vectors. They form icosahedral particles with 240 copies of the trimeric hexon protein arranged on the planes and a penton complex at each of the twelve vertices. The penton consists of a pentameric base, implicated in virus internalization, and a protruding trimeric fibre, responsible for receptor attachment. The fibres are homo-trimeric proteins containing an amino-terminal penton base attachment domain, a long, thin central shaft and a carboxy-terminal cell attachment or head domain. The shaft domain contains a repeating sequence motif with an invariant glycine or proline and a conserved pattern of hydrophobic residues. Here we describe the crystal structure at 2.4 A resolution of a recombinant protein containing the four distal repeats of the adenovirus type 2 fibre shaft plus the receptor-binding head domain. The structure reveals a novel triple beta-spiral fibrous fold for the shaft. Implications for folding of fibrous proteins (misfolding of shaft peptides leads to amyloid-like fibrils) and for the design of a new class of artificial, silk-like fibrous materials are discussed.