An adenovirus was detected by electron microscopy in tissues from falcons that died during an outbreak of inclusion body hepatitis and enteritis that affected neonatal Northern aplomado (Falco femoralis septentrionalis) and peregrine (Falco peregrinus anatum) falcons. Molecular characterization has identified the falcon virus as a new member of the aviadenovirus group (M. Schrenzel, J. L. Oaks, D. Rotstein, G. Maalouf, E. Snook, C. Sandfort, and B. Rideout, J. Clin. Microbiol. 43:3402-3413, 2005). In this study, the virus was successfully isolated and propagated in peregrine falcon embryo fibroblasts, in which it caused visible and reproducible cytopathology. Testing for serum neutralizing antibodies found that infection with this virus was limited almost exclusively to falcons. Serology also found that wild and captive peregrine falcons had high seropositivity rates of 80% and 100%, respectively, although clinical disease was rarely reported in this species. These data implicate peregrine falcons as the natural host and primary reservoir for the virus. Other species of North American falcons, including aplomado falcons, had lower seropositivity rates of 43 to 57%. Falcon species of tropical and/or island origin were uniformly seronegative, although deaths among adults of these species have been described, suggesting they are highly susceptible. Chickens and quail were uniformly seronegative and not susceptible to infection, indicating that fowl were not the source of infection. Based on the information from this study, the primary control of falcon adenovirus infections should be based on segregation of carrier and susceptible falcon species.