Thermoregulation in S. frequens was investigated throughout its adult life, from emergence to reproduction. Data were analyzed by the least squares method. Body temperature was highly correlated to ambient temperature in the sun in the teneral stage and as the life stage advanced the correlation of body temperature to ambient temperature was lowered. Sexual differences were confirmed on thermoregulation; in the male the correlation decreased, whereas, in the female it decreased less. Males have lower body temperatures at high ambient temperatures than the females, and when ambient temperatures were low at the reproductive period in autumn, they maintained high body temperatures. The sexual difference of Tb ranged 2.5-3.4°C. This seem to be due to the sexual difference in body size. Females have smaller thorax and are likely to be influenced by ambient temperature. Because of the small thorax, females seem to be tolerant of high ambient temperature. High body temperature in ovipositing males were not caused by metabolic heat production at the tandem flight, but by the elevation of it during copulation. At low ambient temperatures males were seen warming up at the last stage of copulation. Body temperature in non-contact ovipositing females approximated to that of ovipositing males in tandem and the duration of the former was shorter than that of the latter. 5. frequens, in spite of small size and disadvantage for warm-up, is a periodic endotherm.