To determine whether the agent of Lyme disease, Borrelia burgdorferi, may be maintained in the absence of its usual white-footed mouse reservoir host, Ixodes dammini ticks from an island where mice are absent were examined. Prevalence of spirochetal infection was described for ticks removed from mammals, birds, and vegetation on Monhegan Island, Maine. Forty percent of adult I. dammini removed from vegetation were infected. Norway rats were heavily infested with ticks, and > 60% of such ticks contained spirochetes. Other hosts were less frequently infested by ticks, and few such ticks were infected by spirochetes. The prevalence of antibody to B. burgdorferi was 23% in dogs and cats; 4% of island residents had Lyme disease. Thus, rats maintain Lyme disease spirochetes on Monhegan Island, and there may be transmission of this agent by I. dammini to island residents and their pets.