Influenza A viruses isolated from the cloaca of naturally infected feral ducks replicate in the lungs and in the cells lining the intestinal tract of feral and domestic ducks. Despite the low pH of the gizzard, the duck influenza viruses reach the intestines via the digestive tract and are found in high concentration in the feces. The viruses retain infectivity in fecal material for at least 30 days at 4° and for 7 days at 20°. The morphology of one strain of intestinal duck influenza virus (Hav7 Neg2) that had never been passed in embryonated eggs and was isolated from the feces was roughly spherical and fairly uniform in size and shape. However, another strain of duck influenza virus studied (Hav3 Nav6) was predominantly filamentous, suggesting that the morphology of influenza viruses in their natural hosts varies from strain to strain. After passage in the chick embryo each strain retained the morphological characteristics found in the feces. In contrast to duck influenza viruses, representative human influenza viruses of the HON1, H3N2, and Hswl Nl subtypes replicate only in the upper respiratory tract of ducks. The duck influenza viruses are more stable to low pH than human strains and retain infectivity for over 30 days in nonchlorinated river water at 0° and for 4 days at 22°. The susceptibility of ducks to infection with human and avian strains of influenza virus and the possibility of transmission to animal species through the water supply suggests that ducks may be important in the ecology of influenza viruses. The possibility of "intestinal influenza" virus vaccines is considered. © 1978.