An endemic focus of Lyme disease is present in Colt's Neck, Howell, Freehold, and Wall Townships in Monmouth County, New Jersey. Cases of Lyme disease have occurred in this area from 1978 to 1982. Fifty-seven of the 117 persons (49%) who acquired their infection in New Jersey from 1978 to 1982 live or work in these four townships, whose population of 82,491 is only 1.1% of the population of the entire state. Thirty persons who contracted Lyme disease were exposed to ticks at the Naval Weapons Station, Earle, which is located within Colt's Neck and Howell Townships. The annual incidence rate for persons stationed at or working on this military facility in 1981 and 1982 was about 1%. At Naval Weapons Station, Earle, persons working outdoors in 1981-1982 were at higher risk than those working indoors.