Much still remains to be understood about ecology of Borrelia burgdorferi which is transmitted by a number of ticks belonging primarily to the Ixodes complex, known to feed on different animal hosts and humans. This article pretends to be a review of previous knowledge of Borrelia burgdorferi ecology. Special emphasis is placed on the preliminary results of an investigation conducted in Yugoslavia on the risk of developing lyme borreliosis after a tick bite (Ixodes ricinus). Lyme borreliosis was studied prospectively in 147 persons with tick fastened on them. Dark-field microscopic examination of tick mitgut revealed that 11 (7.9%) od 147 Ixodes ricinus were infected with Borrelia burgdorferi. Lyme borreliosis was demonstrated in only one (0.6%) of all persons bitten by ticks infected with Borrelia burgdorferi. In conclusion, these preliminary data indicate that in Yugoslavia lyme borreliosis infrequently occurred after a tick bite.