During August 1971 three horses died and one human infant became ill during an outbreak of Eastern equine encephalomyelitis in Upstate New York. Fifteen of 93 wild avian sera collected in the area reacted in hemagglutination-inhibition (HI) tests with Eastern equine encephalomyelitis (EEE). EEE virus was isolated from four avian specimens (White-throated Sparrow, Towhee, Baltimore Oriole, Cedar Waxwing) and 5 of 159 arthropod pools (3 from Culiseta morsitans and 1 each from Culex territans and Culex restuans). Sera from 54 humans and 89 healthy equines were HI-negative for EEE. It is believed that southward moving Baltimore Orioles and White-throated Sparrows carried the virus into the area from an unknown location to the north. Virus circulation in the avian population is attributed to C. morsitans. The vector to equines and humans is unknown.