The complete nucleotide sequences for four adenoviruses-Simian Adenoviruses 21, 22, 23, and 24, originally isolated from chimpanzees, were determined. The genome organization of the chimpanzee adenoviruses was found to be similar to that of other adenoviruses. The viral gene products of the adenoviruses Simian Adenoviruses 22, 23, and 24 are very closely related to those of the (previously sequenced) chimpanzee adenovirus Simian Adenovirus 25. Simian Adenovirus 21 is most similar to human subgroup B adenoviruses HAdV-3, HAdV-7, and HAdV-35. Analysis of the capsid proteins hexon and fiber of the chimpanzee adenoviruses also supports the placement of Simian Adenovirus 21 in subgroup B and Simian Adenoviruses 22, 23, and 24 in subgroup E.