When Borrelia burgdorferi B31 low passage strain spirochetes were directly injected into the hemocoel of Dermacentor variabilis (Say) females, the bacteria were cleared from the hemocoel within < 24 h. Viable spirochetes were not found in hemolymph, salivary gland, or ovary tissues by subculturing or by IFA. The hemocyte population increased approximately 6 times within the first 6 h after inoculation compared with the uninoculated controls. In contrast, the soluble total hemolymph protein content decreased inversely with the increase in hemocytes. Borreliacidal activity was demonstrated with cell-free hemolymph from D. variabilis. In vitro antimicrobial assays using hemolymph from borrelia-challenged and nonchallenged ticks resulted in 72% spirochete reductions compared with only 11.5%, respectively, within 1 h. Additional evidence of induced antimicrobial hemolymph protein activity was demonstrated by SDS-PAGE, which revealed upregulation of a lysozyme-like peptide (approximately 15 kDa) (22% increase) and the induction of a approximately 5.8 kDa peptide in the B. burgdorferi-challenged ticks. In contrast with the nonvector borne Bacillus subtilis, D. variabilis presented a rapid and robust response to challenge with cultured B. burgdorferi spirochetes and lead to their early elimination. The role of the tick immune system, including possible differences between vector and nonvector ticks, in determining the success of invasive bacteria is discussed.