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The direction finding measurement has enabled us to estimate the absolute intensity of whistlers at low latitudes. Based on the long-term automatic measurement since December 1981 at Yamaoka (geomagnetic latitude 25°N), nine events are selected for daytime (1500–1900 LT) whistlers and four events for nighttime (0300–0900 LT) whistlers. It is found that the daytime whistlers are more intense than nighttime ones, such that the absolute intensity of the maximum occurrence is 100–125 μV/m and the peak value is 225–250 μV/m during daytime, while the corresponding peak value at night is 150–175 μV/m, about a few decibels weaker than the daytime peak value. Finally, we attempt to interpret this day-night asymmetry of the absolute intensity.