Temperate zone bats can use daily torpor as a means of saving energy. Some argue, however, that torpor is costly for both males and females and that individuals should only use it during times of poor foraging conditions. Others hypothesize that the costs are greater for females and that males should enter torpor more regularly. We tested these alternative hypotheses by using temperature-sensitive radiotransmitters to monitor use of torpor and foraging by free-ranging big brown bats (Eptesicus fuscus). During the pregnancy period, males used torpor at night more and foraged less often than did females. Males also went into deep torpor more often and remained in torpor longer than did females. When they foraged, males and females were away from the roosts for equal periods of time. During the lactation period, males and females rarely failed to forage and foraging times were again no different between the sexes, although males may roost at night away from the maternity colonies. Males again used torpor and deep torpor more often and for longer than females did. These results support the hypothesis that the fitness costs of using torpor are lower for males than for reproductive females and that males regularly use torpor as an energy-saving mechanism. Females enter torpor only when foraging conditions are poor, presumably because torpor prolongs gestation and slows neonatal growth thereby leaving less time for females and their young to prepare for hibernation.