Small rodent species, as well as their ectoparasitic fauna, and the presence of Borrelia burgdorferi in their blood and internal organs (liver and spleen) was investigated, at two locations (Kosutnjak, Lipovicka suma) in Belgrade. Animals were caught between September 15-17, 1992. On location Kosutnjak two Apodemus flavicollis were trapped and Borrelia burgdorferi was isolated from the spleen of one of them. On location Lipovicka suma 13 rodents were trapped as follows: 10 Apodemus flavicollis, 2 Apodemus agrarius and one Mus musculus. In these the presence of Borrelia burgdorferi was not determined. Ectoparasits Ixodes ricinus, Nesopyllus fasciatus and Haemogamasus spp. have been found. The location Kosutnjak can be considered as an active focus of lyme borreliosis.