Abstract: A virus WR strain was isolated from the ducks,which experienced abrupt reduction in egg-laying or died of ovarianhemorrhage.The Muscovy ducks inoculated with the WR strain showed pathological effects similar to that of the field cases.The virus could be recovered from the tissues of the sampled ducks in the wild.The viron of the isolated virus had an enveloping membrane and was 30-50 nm in diameter,as observed under electron microscope.The virus WR strain was sensitive to chloroform,and had no haemagglutination activity on chicken,duck or goose sera at pH 7.2.The virus was tested using RT-PCR,and the positive PCR product sequenced and analyzed.The length of the amplified fragment was about 1kb.The sequenced nucleotide had the greatest homogeneity(i.e.,88.7%) with Tembusu Virus at the nucleotide position 9-777.Phylogenetic analysis of Flaviviridae showed the isolated WR strain appeared to share a common ancestor with Tembusu,Louis encephalitis,Israel turkey meningoencephalomyelitis and Ntaya Viruses.However,it seemed to have its own branch on the evolution tree.The results indicated that the isolated WR strain belonged to the genus Flavirirus,but not a member of any of the known groups in the genus.It might be a new Flavivirus species that has not been reported previously.