The authors compared the contribution of white-footed mice (Peromyscus leucopus), chipmunks (Tamias striatus), and meadow voles (Microtus pennsylvanicus) to infection of vector ticks with the Lyme disease spirochete, Borrelia burgddorferi. At one Massachusetts location where Lyme disease is endemic, all three species of rodents were found to be infected. Prevalence of infection, however, varied from 90% for mice, and 75% for chipmunks to just 5.5% for meadow voles. Infectivity of these hosts for larval Ixodes dammini also varied, but mice were found to be the most infective, followed by chipmunks and meadow voles. Density estimates of these three hosts, collected between 1981 and 1986 in three coastal Massachusetts locations, revealed that mice were more abundant than the other two rodents in areas in which ticks were abundant. In addition, mice were infested more abundantly with larval I. dammini than the other two host species. Integrating these results, we determined each species' "reservoir potential," a novel term which describes the relative contribution made by a host species to the horizontal infection of a vector population. The authors' findings demonstrate that, at least in coastal Massachusetts, P. leucopus is the most important small mammal reservoir for B. burgdorferi.