The spontaneous take-off activity of Blastophagus piniperda was investigated at light intensities of 0.I, I, I0, I00, and I000 lux at a constant temperature of 25°C. and at an air humidity of 77 % R.H. The effect of air humidity (o %, 77 %, and (00 % R.H.) on the take-off rate was studied in spring specimens only. At the low light intensities of 0.I and I lux the cake-off rates were clearly lower than at the higher light intensities of I00 and I000 lux. At all light intensities there were marked seasonal differences in the take-off rate. At each light ntensity the rate was clearly higher in the specimens of the new generation in July than in the spring specimens. At the high light intensities of I00 and I000 lux, and at I0 lux in July, the high rate of take-off activity remained relatively constant throughout the I5-minute experimental period. At the low light intensities the take-off rate was very low in the early part of the experiment, but with time the rate increased considerably. Thus the initial inhibitory effect of very low light intensity is rapidly abolished, probably by adaptation. In the beginning of the I5-minute experiments the take-off rate is high at the air humidity of I00 % R.H., low in the dry air of nearly o % R.H., and intermediate at 77 % R.H. After the first 3-minute period these differences are largely abolished.