Since 1997, the goose flocks in some regions of South and East China have experienced severe outbreaks of Newcastle disease virus (NDV) infection. Although measures have been taken to control the disease, it still spread progressively and became a substantial threat to goose farming. As aquatic birds are usually considered to be resistant even to most virulent NDV strains, the epidemics of the disease attracted much attention regarding the characteristics and source of those goose-pathogenic NDV isolates. In the present study, we determined the mean death time (MDT) in embryonated chicken eggs and intraeerebral pathogenicity index (ICPI) in day-old chickens of 11 isolates derived from outbreaks during 1997-2001, investigated the pathologic changes in vivo in geese and ducks, and cytopathic alterations in vitro in cell culture induced by some of these isolates. And then we genetically characterized them on the basis of partial sequence analysis of the fusion (F) and hemagglutinin-neuraminidase (HN) glycoprotein genes. The results demonstrated that all the 11 isolates were velogenic NDV, and it is most likely that these isolates were originated from virulent strains of NDV in chickens of the same period.1 Pathologic changes in vivo and cytopathic alterations in vitro induced by NDV isolates of goose originA total of eleven NDV isolates were derived from outbreaks during 1997-2001. These isolates gave MDTs ranging from 47.4~60h and ICPI values between 1.80-1.91. In the in vivo experiments, five-day old goslings and ducklings were inoculated with one of these isolates, JS/1/97/Go, through both oral and ocular routes. Marked gross lesions and severe histopathic changes were observed in most organs in the infected goslings. The gross lesions were mainly found in proventriculus, intestinal tract,pancreas, spleen, bursa of Fabricius, characterized by edema of the proventriculus, hemorrhage and necrosis of the intestinal mucosa, multiple necrotic foci in the pancreas and spleen, and atrophy of bursa. Microscopically, extensive celluar damage was the most consistent lesion in all of these tissues. There was severe coagulation necrosis in intestinal epithelium, sometimes with an abundance of celluar debris and erythrocytes. Lymphoid tissues, especially those in spleen, bursa, and cecal tonsil, showed marked loss of lymphocytes. In pancreas, different sizes of necrotic foci could often be observed. In addition, the deciliation and necrosis in trachea epithelium, vacuolar degeneration and necrosis of hepatocytes, granular degeneration and necrosis of renal tubule epithelium, were also primary lesions. However, no obvious gross lesions were seen in the infected ducklings, although mild to moderate histopathological lesions were found in trachea, kidney and bursa of Fabricius.By using immunohistochemical (IHC) staining for the detection of NDV F protein, we investigated the localization and distribution of the viral antigen in tissues from the infected birds. In goslings, the viral antigen could be detected as early as 6h postinoculation (PI). On day 4 PI, there was staining in most of the tissues, except brain and heart. On days 5 through 15 PI. positive staining was consistently present in these tissues with stable intensity. After that, staining waned gradually, and at day 29 PI, most of the tissues except intestine contained no detectable virus antigen. In those positively stained tissues, staining was localized in cytoplasma of lymphocytes, reticulocytes, macrophages. hepatocytes as well as various epithelial cell types. The most intense staining was observed in intestine epithelium and bursa lymphocytes, whereas only sporadic staining was seen in ureter epithelial cells in kidney. In ducklings, several tissues, including trachea, lung, esophagus, proventriculus, bursa of Fabricius, kidney, thymus and Harder's gland, exhibited positive reaction in IHC staining. On days 1 to 15 PI, there was staining in most of these tissues. And on day 19, only the tracheas and kidneys exhibited positive staining. The st