The thermoregulatory and metabolic responses to ambient temperature of Saccopteryx bilineata and the microclimate in three daytime roosts were studied in lowland tropical forest in Costa Rica. Between 20 and 30 C the animals usually remained normothermic (body temperature = 35.5 ± 1.2 C), and their oxygen consumption ($\dot{V}O_{2}$) was inversely related to the ambient temperature: $\dot{V}O_{2} (ml O₂/g·h) = 12.26-0.35 T_{a}$ (°C). The mean minimal heat transfer coefficient was 0.32 ± 0.03 ml O₂/g·h·°C, which is similar to the value expected on the basis of body mass. The mean basal rate of metabolism was 1.86 ± 0.55 ml O₂/g·h, or 91% of the expected rate. Air temperatures inside the tree roosts were very stable, between 25.6 and 27.5 C, while air temperature outside the trees fluctuated between 25.9 and 30.3 C. The basal rate of metabolism of S. bilineata stands in contrast to the consistently low rates (46%-64% of expected) that have been measured on other tropical insectivorous bats. This may be due to the more reliable food source that this bat selects, whereas remaining normothermic at the roost may lower predation risks and allow social interactions.