Body temperatures of winter-resident Korean bats typically range from 10 degrees to 40 degrees C between August and September and from 3 degrees to 15 degrees C between January and April. To learn how behavior and the motor systems of heterothermic bats respond to this body-temperature variation, we examined whole-organism performance and the temperature-dependence of contractile properties of flight muscle in Murina leucogaster ognevi. In winter and midspring, the lowest limits of body temperature were 8 degrees C for biting and crawling, 16 degrees C for visually observable shivering, 22 degrees C for wing flapping (without powered flight), and 28 degrees C for aerial flight. In summer, the lowest temperature limits changed little for biting and wing flapping, but the temperature limits increased about 3 degrees C for crawling, shivering, and flight. Maximum isometric tetanic tension of the isolated biceps brachii muscle was almost insensitive to tissue temperatures between 10 degrees and 40 degrees C, with an average temperature coefficient of 1.02 in summer and of 0.96 in winter. Rate of tetanic tension production between 10 degrees and 40 degrees C and shortening velocity and power between 15 degrees and 25 degrees C were temperature sensitive, with average temperature coefficients of 1.3-2.3. Seasonal differences in contractile properties within each temperature were not significant, except for maximum tetanic tension at 30 degrees - 40 degrees C. Thus, the motor system of the bats had functional capacity over the range of body temperature experienced in winter to summer. The temperature-dependence of behavior was consistent with muscle physiology. The defensive behaviors, like biting and crawling, observed at 8 degrees - 12 degrees C body temperature could be exerted by using temperature-independent tetanic tension, whereas activities, such as flight, that require power generation would be restricted to higher body temperatures by temperature-sensitive rate properties. Some rate processes appeared to be more temperature sensitive in summer than in winter.