Newcastle disease virus was isolated from the cloaca of 1-5% of live-trapped waterfowl in Wisconsin in the fall from 1978-1980. Antibody to NDV was detected in 8% of the birds tested, with no apparent difference between sex and age classes. Experimental infection resulted in persistence of virus shedding for months after exposure. Lack of detectable antibody in some of the experimentally infected birds suggests that reported antibody prevalence may not be indicative of the true prevalence of the infection. Isolation of NDV for the last 9 years as well as the detection of antibody in waterfowl over 25 years ago, suggests a well-adapted host-parasite relationship. Experimental evidence of virus persistence in individual mallards (Anas platyrhynchos) provides a mechanism for maintenance of the virus in the wild population.