Newcastle disease virus (NDV) hemagglutination inhibition (HI) antibody titers of 1:20 or greater were detected in 80 of 200 serums from adult female and 9 of 35 adult male mallard breeders tested from three commercial game farm flocks in Wisconsin and Illinois. Six of 106 serums from mallard ducklings also were considered to be reactors. Almost half (49) of 100 swan serums from five aviculturist flocks in Michigan had HI antibody titers of 1:20 or greater. The possible significance of these findings regarding waterfowl management practices was discussed.