The course of Borrelia burgdorferi-infection in Columbian black-tailed deer (Odocoileus hemionus columbianus), its effect on the health of these animals, and their reservoir competence for fleas were evaluated experimentally. Four yearling females inoculated intramuscularly with 10⁸ organisms of the CA4 strain of B. burgdorferi, and two yearling males unexposed to spirochetes, were monitored daily for 3 mo. Spirochetes were reisolated from the blood of three does at 14 or 70 days postinjection, and from several tissues of the fourth doe at necropsy. Considerable antigenic heterogeneity was observed among the reisolates as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Only two of the four infected deer developed significant antibodies (≥ 1:128) to B. burgdorferi with titers persisting for ≤2 mo. Hematological values were highly variable and the degree of variation observed was much greater than that reported previously for Columbian black-tailed deer or other subspecies of mule deer. Infected deer did not manifest signs of Lyme disease. On histologic examination of eight tissues per deer, we observed a minimal hepatic lesion in all animals exposed to B. burgdorferi. No spirochetes were detected in 367 fleas (Pulex irritans) that had naturally infested these deer; thus this flea probably is an inefficient host of B. burgdorferi.