On the basis of «Annals of Nature of Tsentralno-Chernozemny Biosphere Reserve», its colonization by alien mammals species and the current state of their populations have been analyzed. In the mammal fauna of the reserve, 16 out of 50 can be classified as alien. They are distributed in the following manner according to the pathways of penetration into terrestrial ecosystems: 1) disseminated from the sites of introduction (musk rat, raccoon dog, American mink, and red deer); 2) self-disseminated species (serotine bat, European beaver, Eurasian red squirrel, bobak marmot (also re-introduced), European pine marten, beech marten, wild boar, European roe deer, and moose); 3) accidentally introduced (house mouse, brown rat and feral dog). Intentionally introduced species are absent on the territory of biosphere reserve.