Czechoslovakia, fortunately enough, belongs to the regions having still numerous habitats resembling the original ones, or at least suitable to the needs of many original species of mammals. This concerns especially the bats while these animals are partly adapted to a synanthropic way of life in a country with many old buildings, parks, woods of different types, etc. Therefore, the protection of bats in Czechoslovakia appears not to be such a serious problem as in some other central and western European countries. Apart from the region of the South Slovakian Kars, we have no data on the decline of bat populations as a whole. Considering this, we were interested in the influence of our own activities on the populations in certain localities. This activity concerned banding, recapturing, and also collecting of bats for the purpose of systematic, morphological, parasitological, and physiological investigations. Figs. 1 and 2 show the fluctuations in number of several species of bats recorded in two bat hibernating quarters in Bohemia. Both of these localities are situated in forested areas and populated by such species as Plecotus, Barbastella, and Eptesicus nilssoni — all of them showing preference for cooler hibernating quarters.