White-tailed deer (Odocoileus virginianus) were examined for the tick, Ixodes dammini, and sera were analyzed for antibodies to spirochetes during 1982. Of the 323 animals inspected in four areas endemic for Lyme disease, 188 (58%) had adult ticks; parasitism ranged from 43% at Haddam to 82% at East Lyme. Direct and indirect fluorescent antibody tests detected spirochetes in 18 of 133 (14%) ticks. Indirect immunofluorescence tests revealed antibodies at titers of 1:64–1:4,096 to this bacterium in 93 (28%) of the 332 sera assayed. There is a close correlation among the distribution of spirochete-infected I. dammini, deer with antibodies, and human cases of Lyme disease.