We tested the 5% rule for the ratio of radiotransmitter mass to body mass by applying radiotransmitters and passive integrated transponders (PIT tags) or PIT tags alone to adult, female big brown bats (Eptesicus fuscus) roosting in buildings in Fort Collins, Colorado. We used records from PIT readers at roosts to compute apparent annual survival of both groups from 2001 to 2003 and found them to be similar. All bats examined 1 year after radiotagging were reproductively active and had body masses similar to bats not radiotagged. Big brown bats do not appear to suffer from major long-term effects of carrying transmitters within the 5% rule.