1. It was clarified, by means of breeding experiments conducted within a temperature range of 14-34°C, if the growth of Pieris influenced by diurnally fluctuating temperatures depends on a simple additive influence of day-and-night-temperatures or if its growth is accelerated or retarded under these conditions by special physiological adaptations. 2. On the assumption of an additive temperature influence, the expected growth values of fluctuating temperatures can be calculated by the growth values of constant temperatures. In linear relations between developmental speed and temperature the expected values correspond with the values of the mean constant temperature. In sigmoidal relations one may expect that the growth speed in fluctuating temperatures will increase in the lower temperature range and decrease in the upper range. 3. Breeding of Pieris brassicae in diurnally fluctuating temperatures (amplitude 8°C) resulted in good conformity with the growth values expected above. Thus, different day and night temperatures act in a simple additive way. 4. The larvae of the Pieris stock had been eating all the time during the day and night. It is suggested that special adaptations to diurnally fluctuating temperatures might exist in insects with daily behavioural rhythms.