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able shelters determines the occurrence or absence of field mice in an urban green space. The number of shelters may also be assumed to de termine population numbers. The large number of shelters rich lux uriant vegetation and in the surface layer of the soil, and the very rare visits to the area by dog’s and humans, justify the assumption that the cemetery area is among the most abundant in numbers of A. agrarius in Warsaw. Acknowledgem ents: The au tho r wishes to thank Dr. R. A ndrzejew ski, Dr. J. Ba-binska-W erka and Dr. J. Gliwicz for the ir assistance in these studies and th e ir helpful com m ents on the tex t of this paper