The penton capsomer of mammalian adenoviruses consists of a trimeric, long and thin fibre inserted into a pentameric base. The avian adenoviruses possess a penton which presents another symmetry mismatch: each pentameric base is associated with two fibres. Here we have studied the morphology of the penton of CELO virus, an avian adenovirus, and we have determined the sequence of both fibres, one long and one short. The short fibre is probably associated with the base in the same way as the mammalian viral fibres and we will discuss how the long fibre could be attached. The shafts of all known adenovirus fibres consist of a series of 15-residue repeats. The avian virus fibres show a more complicated and less regular shaft repeat structure with single, double and triple repeats. The sequences of the receptor binding (head) domains of both fibres are very different from all other known fibre head domains and very different from each other, suggesting that the two fibres might bind to different receptors. The genome organization of the sequenced region is rather different from that in human adenoviruses. In particular, a region homologous to the human virus E3 region was not found at the position where it normally occurs in the human virus genome.