The infectivity of a diverse collection of Borrelia burgdorferi strains from North America for mice was determined as a prelude to vector competence experiments with the 3 primary human-biting tick species in the eastern United States (Ixodes scapularis Say, Dermacentor variabilis (Say), Amblyomma americanum (L.)]. Of the 34 B. burgdorferi strains inoculated into mice, 29 were infectious; the exceptions were 5 isolates from Texas. Vector competence experiments were conducted with 2 strains from the southern United States (North Carolina and Georgia). Both strains were extremely infectious to I. scapularis larvae. Moreover, I. scapularis efficiently maintained these spirochetes transstadially and transmitted infection as nymphs. D. variabilis larvae were intermediate in susceptibility but generally did not maintain the infection transstadially. A. americanum larvae were completely refractory to infection with these 2 southern B. burgdorferi strains. Three isolates from Michigan D. variabilis were inoculated into mice, subsequently exposed to I. scapularis and D. variabilis larvae. Larval I. scapularis were 5-fold more susceptible to infection with these strains than were larval D. variabilis. Although nymphal I. scapularis efficiently transmitted a Michigan isolate, nymphal D. variabilis did not. In all these experiments, I. scapularis was the only species that proved to be vector competent for B. burgdorferi.