Abstract 1. 1. A zone of thermoneutrality exists for adult female Myotis thysanodes in early September in an ambient temperature range of 32·5–34·5°C with minimum metabolism of 1·74 cm 3/g per hr. 2. 2. A transition from a homeothermic to a non-homeothermic metabolism to ambient temperature relation is evident by late September in adult female M. thysanodes. 3. 3. In early September, some adult female M. yumanensis exhibit a homeothermic metabolism to ambient temperature relation while others exhibit a non-homeothermic relation. 4. 4. In early September, adult M. lucifugus occultus of both sexes exhibit a non-homeothermic relation. 5. 5. At ambient temperatures above 32·5°C, metabolism in M. thysanodes increases at a significantly more rapid rate than in M. lucifugus occulttus. 6. 6. A critical thermal maximum ambient temperature of 44·5°C has been found for each of these three species.