Although only 2 bat species are common in the Greater Fundy National Park Ecosystem of New Brunswick, Canada, 4 distinct groups occur in terms of roost-site selection due to sexual segregation. We found that female northern long-eared bats (Myotis septentrionalis) roosted alone or as part of maternity colonies in mature, shade-tolerant deciduous stands. A roost site was 24 times more likely to be in a shade-tolerant deciduous tree than a coniferous tree, and trees in the mid-decay classes were 5.2 times more likely to be used than live or recently dead trees. Male northern long-eared bats roosted alone in coniferous stands or conifer-dominated mixedwood stands, and they selected roost trees that were in the mid-decay stages. Trees in mid-decay stages were selected more than twice as often as trees at the early or late stages of decay. For every 10% increase in the proportion of coniferous trees in the plot, the chances of a male northern long-eared bat roosting in the plot increased 1.5 times. Female little brown bats (M. lucifugus) in our study area were transients. Male little brown bats primarily roosted alone in coniferous stands, or conifer-dominated mixedwood stands. Their selection of roost sites was highly dependent on the number of snags in the vicinity. For every increase of 5 snags in a 0.1-ha plot, the odds of use for male little brown bats on that site increased by 2.5 times. The long-term availability of roost sites for males of both species in our study area may be linked to spruce budworm (Choristoneura fumiferana) epizootics, which is the major mortality factor for coniferous species in the region. Female northern long-eared bat roosts probably are more consistantly available because suitable sites are created by continuously occurring, individual-tree stressors in mature shade-tolerant forests.