I measured the metabolic rate (MR) of four male southern forest bats (Vespadelus regulus; 5.5 g) exposed to a diurnal increase in air temperature (T_a) from 13 to 26°C, simulating conditions in natural tree roosts. Three bats remained in torpor throughout the day, despite the rise in T_a, whereas one bat aroused at a T_a of 25.2°C and was normothermic for 108 min until re-entering torpor when T_a declined in the afternoon. All bats aroused shortly after lights off. Torpid MR increased exponentially with rising T_a, yet even at 26°C remained only 16% of minimum resting MR at the same T_a. Rest-phase energy expenditure (12 h), including the estimated cost of an evening arousal, ranged from 0.62 to 1.23 kJ. Thus, torpor provides these small bats with an enormous reduction in energy consumption even at T_a close to their thermoneutral zone.