Brush rabbits (Sylvilagus bachmani) and black-tailed jackrabbits (Lepus californicus) from California (USA) were assayed for antibodies to Borrelia burgdorferi, the etiologic agent of Lyme borreliosis. Significant antibody titers were detected in 90% (range, 67 to 100%) of brush rabbits from four of six localities, and in 90% of jackrabbits from a single locality in northern California. One of the populations of brush rabbits that did not yield seropositive individuals inhabited an oceanic island devoid of any other terrestrial mammal, whereas the other population was located on an isolated flood plain bordering San Francisco Bay. Absorption tests using B. burgdorferi as antigen revealed that antibodies detected in both species of lagomorphs were directed against borreliae. These findings reinforce the earlier suggestion that lagomorphs may be useful as sentinel animals for surveillance of borreliosis in the far western United States.