The structure of woodland bat communities is influenced by numerous environmental factors, and amongst these, the availability of suitable roosts is of prime importance. Temperate zone forest-dwelling bats use a great variety of roost types, ranging from natural tree cavities to human-made shelters. Given the frequent habit of forest bats to switch roosts, even within the reproductive season, bat-friendly forest management requires information about the number of cavities necessary to maintain populations. We identified the rate of roost switching, number of roosts used and site characteristics of two forest bat species at risk, the Bechstein's bat (Myotis bechsteinii) and the Barbastelle bat (Barbastella barbastellus) in suburban forests of SW Switzerland. Radio tracking of 9 M. bechsteinii females showed that a colony used at least 15 roost sites in an area of 3 km(2) throughout the reproductive season. B. barbastellus used at least 11 roost sites located in France in two areas 15 km from each other. This illustrates the borderless nature of bat conservation and calls for the maintenance of a trans-frontier cooperation programme. There were clear species-specific roost preferences: M. bechsteinii used tree cavities whereas B. barbastellus used exclusively human-made shelters. These results provide some preliminary guidance for bat-friendly forest management.