Data reported to CDC through the West Nile Virus (WNV) Surveillance System have shown an increase in the geographic range of WNV activity in 2000 compared with 1999, the first year that WNV was reported in the Western Hemisphere. In response to this occurrence of WNV, 17 states along the Atlantic and Gulf coasts, New York City, and the District of Columbia conducted WNV surveillance, which included monitoring mosquitoes, sentinel chicken flocks, wild birds, and potentially susceptible mammals (e.g., horses and humans). In 1999, WNV was detected in four states (Connecticut, Maryland, New Jersey, and New York) . In 2000, epizootic activity in birds and/or mosquitoes was reported from 12 states (Connecticut, Delaware, Maryland, Massachusetts, New Hampshire, New Jersey, New York, North Carolina, Pennsylvania, Rhode Island, Vermont, and Virginia) and the District of Columbia. Of the 13 jurisdictions, seven also reported severe neurologic WNV infections in humans, horses, and/or other mammal species. This report presents surveillance data reported to CDC from January 1 through November 15.