Energy utilization and water requirements of captive Myotis thysanodes and Myotis lucifugus (Chiroptera).

l. Ten captive Myotis thysanodes ingested an average of 5.05 kcal/ day and egested 0.47 kcal/day with a mean energy utilization of 90.8 per cent. 2. One captive M. lucifugus ingested 4.15 kcaljday and egested 0.37 kcal/day with an energy utilization of 91.2 per cent. 3. M. thysanodes gained an average of 2.77 cm3/day of water and lost an average of 2.86 cma/day. This represents a water turnover of almost half the total body water. 4. The M. lucifugus gained 1.97 cm3/day of water and lost at least 1.47 cm3/ day representing a water turnover of about one third of the total body water. 5. It is suggested that for small mammals in general that the energy utilization approaches or exceeds 90 per cent efficiency.