The giant noctule is a rare European bat about which we have little ecological information, but it is a priority species for which we need to determine the best conservation measures. Acoustic monitoring followed by a radio-tracking survey resulted in the localisation of two maternity colonies in different forest areas in the Western Carpathians, one in protected mixed stands in a national park (Muránska planina Mts) and the other in disturbed secondary spruce forests (Balocké vrchy Mts). The 95% home-range polygon of the roosts was about 3.5 km² for both colonies. Eurasian aspen (Populus tremula) accounted for 100% of roosts (n = 18 trees) in mixed stands. However, even in the spruce monocultures, bats roosted in cavities of this tree whenever possible, although 76% of roosts (n = 21 trees) were still in Norway spruce (Picea abies). Regardless of forest type, the bats preferred to roost at the edges of the stands and on steep slopes that offered a free exit from the roost into open space. Small-scale salvage logging during bark beetle outbreaks can also create suitable open roosting habitats. Our observations indicate that structured stands with a higher proportion of aspen will positively affect tree-dwelling bats in mountain forests.