Diagnostic investigations were carried out in a fatal disease of several African Grey Parrots (Psittacus erithacus) and Cape Parrots (Poicephalus robustus) in a large breeding plant. Electron microscopically examination of liver and intestine, the organs with the most prominent pathomorphological changes, regularly revealed adenoviruses. Necrotizing hepatitis and catarrhal to haemorrhagic enteritis dominated on histopathological examination. Numerous basophilic intranuclear inclusion bodies in hepatocytes and enterocytes and their ultrastructural characteristics underline the etiological role of the detected adenoviruses. Adenoviruses were isolated from livers of three different birds and once from the intestine. Serologically the isolates were classified as fowl adenovirus serotype 4 (FAV4). Restriction enzyme analysis of two isolates showed the identity with the FAV4 reference strain KR5.