A total of 472 field-collected Ixodes ricinus ticks from southern Germany were investigated by immunofluorescence for the presence of Borrelia burgdorferi with a polyvalent rabbit immune serum and with monoclonal antibodies specific for outer surface proteins A and C (OspA and OspC, respectively). Borreliae were detected in 90 ticks with the polyvalent immunofluorescence assay. Infection rates in adults (females, 20.2%; males, 25.2%) were significantly higher than in nymphs (12.1%). OspA was detected in 77 ticks and OspC was detected in only 1 tick with the respective monoclonal antibodies. We therefore conclude that B. burgdorferi in unfed I. ricinus ticks usually expresses OspA and very rarely OspC.