Phylogenetic comparisons of adenovirus DNA sequences, including the recently completed genomic sequences of Ad40 and Ad12, have been performed in order to investigate the evolutionary relationships among the various serotypes. Phylogenetic trees were constructed from sequence data for the ITR, E1a, E1b, E2a, E3b, major late promoter, hexon, protease, and fiber regions of the genome using programs contained in the PHYLIP (Phylogeny Inference) package. In general the branching pattern of the human serotypes at each locus correlated well with the classification of the human serotypes into six subgenera (A-F). However, a close evolutionary relationship was inferred between Ad4 (the only member of subgenus E) and the subgenus B viruses Ad3, Ad7, and Ad35, and challenges the placement of Ad4 in a subgenus of its own. In addition, the human viruses of subgenera A (Ad12, Ad18, and Ad31) and F (Ad40 and Ad41), as well as the simian adenoviruses SAV16 (SA7) and SAV8 (SV30), all of which are associated with infections of the gastrointestinal tract, were found to cluster together. The results suggest that these viruses have followed a course of evolution distinct from those of the other subgenera which largely infect the respiratory tract. Analysis of genetic variability between the four complete genomic sequences (Ad2, Ad5, Ad12, and Ad40) identified three regions subject to more rapid change, corresponding to the hexon-, fiber- and E3a-coding regions. Genetic variability at the E3a locus is particularly striking and may relate to the pathogenicity of the various serotypes.