1. Two series of hourly measurements of temperature and light intensity, each extending over 24 hours, were made in artificial pools. One pool contained clear water, one muddy water, one standing grass 1 ft. high and one was completely shaded by papyrus matting at a height of 1 ft. Shade temperature records were kept in a Stevenson screen.

2. The results showed that the grass had a pronounced insulating effect on temperature. The range in the grassy pool was less than that in the screen and less than half than that in the open pool. The grassy pool was warmer by night and cooler by day than the open pool. No essential difference was noted between the temperature cycle of the clear open pool and the muddy open pool nor between the grassy pool with broken shade and the pool receiving complete shade from matting. Maximum light intensity in the grassy pool was less than half that attained in the open pool.

3. It is concluded that not only do the breeding waters of A. gambiae reach much higher temperatures than those of such species as A. funestus, A. coustani and A. pharoensis, but that they also undergo much greater temperature fluctuations.