The effect of aerially sprayed 5 per cent. DDT solution against Anopheline larvae and adults has been investigated.

An account is given of a rapid field method for estimating the concentration in milligrammes per square metre.

At a concentration of 2–3 mg. per square metre, aerially sprayed DDT solution had little effect on Anopheline larvae breeding in water completely covered with a species of Limnanthemum. Also at this concentration tall grass gave partial protection, whereas in open water there was a complete kill.

The residual effect of DDT at a concentration of 2–3 mg. per square metre did not last longer than six days.

Under negligible wind conditions, resulting in a vertically falling spray, there was no penetration into huts even when situated in the open.

Drastic filtration of the spray resulted in a fall in the concentration from 2 to 3 mg. per square metre in the open to less than ·05 mg. per square metre under heavy shade, consequently shrubs at ground level in heavy wooded areas were greatly protected from contamination with DDT and afforded practically safe resting places for adults.