We analyze the frequency of occurrence of small mammals recorded in natural cavities and nest boxes in the Białowieża Forest (eastern Poland) and also describe some parameters of tree cavities. A total of 748 cavities and 60 nest boxes in primeval tree stands and 190 nest boxes in managed tree stands were monitored. Both cavities and nest boxes in primeval stands were occupied by mammals less often than nest boxes in managed stands (0.70%, 0.12%, and 1.26%, respectively). The following mammal species were recorded in natural cavities: Eurasian red squirrel ( Sciurus vulgaris L., 1758), forest dormouse ( Dryomys nitedula (Pallas, 1778)), yellow-necked mouse ( Apodemus flavicollis (Melchior, 1834)), and bats (Microchiroptera species). Three mammalian species were recorded in nest boxes: forest dormouse, fat dormouse ( Myoxus glis (L., 1766)), and yellow-necked mouse. The attractiveness of the boxes for mammals increases in managed forests, probably because of a shortage of natural cavities. We suggest that the role of rodents in the breeding ecology of cavity-nesters is underestimated, since studies on natural cavities are rather rare and the species identities of nest predators are most likely poorly recognized. To understand the breeding ecology of birds, ornithologists should study the population dynamics of mammals and the manner in which they use tree cavities and nest boxes.