A field isolate of Newcastle disease virus (NDV) isolated from northern pintail (Anas acuta) in Tohoku district, northeast Japan, was characterized. Phylogenetic analysis of the fusion protein indicated that the isolate belonged to genotype I and was closely related to isolates from the Far East corresponded to the migration route for this bird species. The isolate had the typical avirulent cleavage site of the fusion protein (112)GKQGR*L(117). In addition, pathogenicity tests indicated the isolate to have avirulent characteristics. However, the isolate has been shown to cause fusion cytopathic effects and form plaques on chicken embryo fibroblasts (CEF) in the absence of trypsin. The present results suggest that the CEF-adapted NDV, which is avirulent, is circulating among waterfowl populations.