Populations of adult Ixodes scapularis and Ixodes pacificus, the two principal vectors of Lyme disease spirochetes in the United States, were collected from 17 sites in 12 states. Female ticks were fed on experimental rabbits; ticks and rabbits were subsequently examined for infection with Borrelia burgdorferi. Fourteen rabbits were exposed to I. scapularis ticks from the northeastern states of Connecticut, New York, New Jersey, and Maryland; all 14 rabbits became infected with B. burgdorferi. A total of 165/226 (73%) of these northeastern ticks was infected. Similarly, ticks from the midwestern states of Michigan, Wisconsin, and Minnesota transmitted infection to all three exposed rabbits; 29/51 (57%) of these midwestern I. scapularis were infected. In marked contrast, none of the 12 rabbits exposed to I. scapularis ticks from the southeastern states of South Carolina, Georgia, Florida, and Mississippi acquired infection with B. burgdorferi, and 0/284 (0%) of these ticks contained spirochetes. Four rabbits were exposed to I. pacificus collected from one location in California; 2/4 of these rabbits acquired infection and 2/57 (4%) of the I. pacificus were infected with B. burgdorferi. The antigenic profiles of all 58 strains tested were consistent with an identity of B. burgdorferi sensu lato. The availability of a human Lyme disease vaccine adds urgency to our efforts to calculate the ecological transmission risk throughout the United States, as an aid to the judicious use of such a vaccine.