Sera from 352 deer from nine New York State counties were tested for neutralizing and hemagglutination-inhibiting antibodies to six arboviruses representing four groups. Antibody titers to California encephalitis and Cache Valley viruses were detected in varying frequency in the 9 counties with foci for the Bunyamwera group in Seneca, Dutchess, and Erie counties. Neutralizing antibodies to western equine encephalomyelitis were noted in sera collected in 1959–61 from Albany and Seneca counties while a focus of group B arbovirus activity, most probably due to Powassan virus, was found in Shelter Island, Suffolk County. Our experience indicates the usefulness of deer as natural indicators of activity of certain arboviruses.