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In 1969 and 1970 calling and chorusing of the tree frog, Hyla a. arborea, was recorded daily, together with air temperature and light intensity. The onset of calling occured during the first days of May at 7.45 p.m. and shifted to about 9 p.m. during June and July. The treefrogs began calling at a light intensity of lower than 260 Lux and chorused at less than 60 Lux, provided the air temperature was above 8°C, which is the lower limit for calling activity. The start of the daily calling activity is controlled by the light intensity and by an annual endogenous rhythm. The effect of the rhythm is 6 to 10 times greater than that of the light intensity.