Three taita falcon (Falco fasciinucha) nestlings and 3 cohoused hybrid falcon (F rusticolus × F peregrinus) nestlings died after showing lethargy and anorexia for 4–5 days. Major lesions in the falcons were necrotizing hepatitis, splenitis, and lymphoplasmacytic nephritis. One hybrid falcon also had a heterophilic nephritis. Intranuclear inclusion bodies were present in the hepatocytes of the taita falcons and in the renal tubular epithelial cells of 1 hybrid falcon. Adenoviral particles were found in the cloacal content of 2 taita falcons. DNA in situ hybridization revealed aviadenoviral nucleic acid within the nuclei of hepatocytes of a taita falcon and a hybrid falcon and in the tubular epithelial cells of a hybrid falcon. Aviadenoviral DNA was amplified by polymerase chain reaction in the liver of taita falcons and hybrid falcons. The death of these birds was attributed to the aviadenovirus infection; however, the source of the virus was not determined. Predisposing factors for the outbreak of the viral infection in these falcons may have included transport stress, lack of quarantine measures, and cohousing of falcon species originating from isolated populations with other falcon species.