Ten avian type A influenza viruses consisting of seven waterfowl-origin, one pheasant-origin, and two turkey-origin viruses were evaluated for their pathogenicity potential after intravenous inoculation into domestic turkeys and mallard ducks (Anas platyrhynchos). The replicative abilities and tissue trophism properties of each virus isolate were examined in both species. The overall virus-isolation rate and histopathological lesion score were greater in the turkeys than in the ducks. The waterfowl-origin viruses caused more tissue damage in turkeys than in ducks but had a narrower tissue distribution range. The pheasant isolate was extremely pathogenic in turkeys but had limited distribution and little effect in ducks. The turkey isolates were more pathogenic in turkeys than in ducks. The pancreas was the most severely affected organ in turkeys, followed by kidney and liver. The spleen and bursa were the most commonly affected organs in ducks.