Adenoviruses can cause infectious diarrheal disease or respiratory infections in humans; 2 recent reports have indicated probable human infection with simian adenoviruses (SAdVs). To assess the possibility of animal-to-human transmission of SAdVs, we tested fecal samples from asymptomatic rhesus macaques housed in 5 primate facilities in the United States and cultured 23 SAdV isolates. Of these, 9 were purified and completely sequenced; 3 SAdV samples from the American Type Culture Collection (SAdV-6, SAdV-18, and SAdV-20) were also completely sequenced. The sequence of SAdV-18 was closely related to that of human adenovirus F across the whole genome, and the new isolates were found to harbor 2 fiber genes similar to those of human adenovirus (HAdV) strains HAdV-40 and HAdV-41, which can cause infectious diarrhea. The high prevalence of adenoviruses in fecal samples from asymptomatic rhesus macaques and the similarity of the isolates to human strains indicates the possibility of animal-to-human transmission of SAdVs.