The genome of adenovirus (AV) types 1, 2, and 5 is known to be very variable as evidenced by the great number of genome types described (36, 61, and 35 for AV1, 2, and 5 respectively). Physical maps were constructed for nearly all of the restriction or R-variants by biochemical methods and by adapting restriction fragments of defined molecular weight. The alterations found with seven DNA restriction endonucleases were mapped on the genome. Altered restriction sites, found for the genome types of each of the three serotypes, appeared to be randomly distributed over the genome, although mutations seemed to occur preferentially on distinct sites of the genome. Many strains, especially those of AV5, were genomically much more related with one another than with the prototype. This finding is compatible with the interpretation that the related strains were derived from an unknown common ancestor.