Adenovirus genomes consist of linear DNA molecules containing inverted terminal repeat sequences (ITRs) of 100 to 200 base pairs. The importance of identical termini for viability of adenoviruses was investigated. The viral strains used in this study were wild-type adenovirus type 5 (Ad5) and a variant Ad2 strain with termini which were distinct from those of all other human adenoviruses sequenced to date. A hybrid virus (sub54), obtained by recombination between Ad2 and Ad5, derived the left 42 to 52% of its genome from Ad2 and the right 58 to 48% from Ad5. Southern blotting analysis with labeled oligodeoxynucleotides indicated that both Ad2 and Ad5 ITRs were present in sub54 viral DNA preparations, and successive plaque purifications of sub54 demonstrated that viruses with nonidentical terminal sequences were viable but were rapidly converted to viruses with identical ends. Cloning of the sub54 genome as a bacterial plasmid supported the observations made by analysis of sub54 virion DNA. A plasmid, pFG154, was isolated which contained the entire adenovirus genome with an Ad2 ITR at the left terminus covalently linked to an Ad5 ITR at the right terminus. Upon transfection of mammalian cells with pFG154, viral progeny were obtained which had all possible combinations of termini, thus confirming that molecules with nonidentical termini are viable. Pure populations of viruses with nonidentical termini could not be isolated, suggesting efficient repair of one end with the opposite terminus used as a template. A model for this process is proposed involving strand displacement replication and emphasizing the importance of panhandle formation (annealing of terminal sequences) as a replicative intermediate.