The sudden outbreak and quick spread of an egg drop syndrome in the major duck-producing regions in China has resulted in serious economic loss. This prompted us to conduct a thorough investigation of the cause of the disease. From preliminary epidemiology, pathogen isolation, and reproduction of the clinical disease by the isolated virus, BYDV, Koch��s postulates were fulfilled, and a new flavivirus causing a serious duck disease was identified. This is the first severe duck disease caused by a zoonotic flavivirus ever reported. It had a very significant NKY80 effect on the duck industry, especially in Southeast Asia where duck farming is most popular and mosquitoes are active. Although the transmission vector of the BYDV was not defined in our study, most of its closely related viruses are transmitted by mosquitoes or ticks. This should be addressed in the future studies. However, it should be noted that the virus infection in egg-laying ducks in this outbreak continued into autumn when mosquito activity is low in northern China. Flaviviruses are notorious for their severe infective and zoonotic nature. Indeed, they can cause serious diseases for both humans and animals. Infections of poultry by flaviviruses are not uncommon, such as West Nile virus and Israel turkey meningoencephalitis virus in geese and turkeys. Further, a newly emerging African flavivirus, Bagaza virus, both causes human illness and infects birds in Africa. However this is the first report of flavivirus infection in ducks with a severe outcome and the future implications of such findings should be heeded. It is noteworthy that, in our initial virus gene amplification screening, a hypothesis for PCR primer design, based on an early report that EEEV causes egg-drop in turkey, helped us have obtained BYDV gene fragments. Our findings revealed that the BYDV is closely related to Tembusu virus. Tembusu virus was MST-312 initially isolated from mosquitoes as early as 1955, but its relevance to human or animal health has never been fully established. However, both the existence of antibodies in humans against Tembusu virus and a new closely-related virus being correlated to chick encephalitis and retarded growth, raise serious concerns about BYDV as an emerging pathogen.