Arthritogenicity, as determined by joint swelling and synovial histology, was compared between or within two Borrelia genospecies that cause Lyme arthritis in humans. The spirochete burden in bladder tissue (a site of spirochete persistence) was documented by quantitative polymerase chain reaction, and immune responses were analyzed. In C3H/HeJ mice, three B. burgdorferi isolates and two of the three B. garinii isolates induced severe arthritis and swelling. Previous designation as invasive or noninvasive B. garinii, or RNA spacer type of B. burgdorferi did not determine arthritis severity induced by isolates. Compared with the other five isolates, the B. garinii PBi isolate induced significantly less arthritis, a lower humoral immune response, and persisted at a much lower level in bladder tissue. However, B. garinii PBi isolates induced similar Borrelia antigen-specific inflammatory T cell responses from the local draining lymph node. Thus, diverse B. burgdorferi and B. garinii isolates were highly arthritogenic in immune competent mice.