Mouse adenoviruses (MAdV) are supposedly the oldest members of the genus Mastadenovirus. Currently, there are three distinct MAdV types known with rather different tropism and pathology. Here we report and annotate the DNA sequence of the full genome of MAdV-2. It was found to consist of 35,203bp thus being considerably larger than the genomes of the other two MAdV types. The increased size of the MAdV-2 genome is generally due to larger genes and ORFs, although some differences in the number of ORFs were observed for the early regions E1, E3 and E4. The homologue of the 19K gene of E1B from MAdV-2 codes for 330 amino acids (aa) and is almost twice as large as from other mastadenoviruses. Accordingly, only the N-terminal half (155aa) has homology to the 19K protein. A homologue of the gene of the 12.5K protein was identified in the E3 region of MAdV-2, but not in MAdV-1 or MAdV-3. The other gene of yet unknown function in the E3 region of MAdV-2 seems to be unique. The E4 region of MAdV-2 contains three ORFs. One has similarity to the 34K gene of other AdVs. Two unique ORFs in the E4 region of MAdV-2 have no homology to any of the five and six ORFs in the E4 region of MAdV-1 or MAdV-3, respectively. Phylogenetic analyses showed that the three murine AdVs have a close common ancestor. They likely formed the first branching of the lineage of mastadenoviruses, and seem to be the most ancient representatives of this genus.