The sequence of the fibre of bovine adenovirus type 3 (BAd3) predicts an extremely long structure due to a large number of 15-residue repeats in the fibre shaft, the tail and head domains being similar in size to the human adenovirus fibres. The length of the fibre was confirmed using negative-strain electron microscopy of BAd3 pentons (fibre plus penton base). The fibre was found to be bent in several discrete places and the bending sites appear to correspond with irregular repeats in the shaft. We suggest that bending of the fibre is needed for the interaction of the penton base with the secondary receptors on the cell surface.