The adenovirus penton, a noncovalent complex of the pentameric penton base and trimeric fiber proteins, comprises the vertices of the adenovirus capsid and contains all necessary components for viral attachment and internalization. The 3.3 A resolution crystal structure of human adenovirus 2 (hAd2) penton base shows that the monomer has a basal jellyroll domain and a distal irregular domain formed by two long insertions, a similar topology to the adenovirus hexon. The Arg-Gly-Asp (RGD) motif, required for interactions with cellular integrins, occurs on a flexible surface loop. The complex of penton base with bound N-terminal fiber peptide, determined at 3.5 A resolution, shows that the universal fiber motif FNPVYPY binds at the interface of adjacent penton base monomers and results in a localized structural rearrangement in the insertion domain of the penton base. These results give insight into the structure and assembly of the adenovirus capsid and will be of use for gene-therapy applications.