1. Certain experiments intended to throw light on problems arising from the practical application of DDT against A. gambiae and A. funestus are recorded. These refer particularly to the action of larvicidal DDT on adult mosquitos; to the effect of varying periods of exposure; and to the possibility of repellence of adults by DDT treatments.

(a) Solutions of DDT on water surfaces produce an appreciable mortality, particularly on laying mosquitos, but also on ♀♀ in earlier stages of ovary development, when confined in cages with a bowl of water so treated.

(b) The greatest effect is produced by a DDT solution in a heavy oil film of long persistency.

(c) The presence of a DDT solution on the water, particularly when applied in a heavy oil, appears to have produced an appreciable inhibition against laying.

3. With a 5-months-old residual film of 200 mgms. DDT per sq. foot on smooth wood, a 60-minute exposure killed 22 per cent. immediately and 100 per cent, within 3 hours. After a 5-minute exposure none was killed immediately, but 17 per cent. were dead within 3 hours, and 73 per cent. at 12 hours. Intermediate periods of exposure gave corresponding results.

4. Anopheles exposed in inter-connecting treated and untreated cages showed characteristic restlessness as a result of contact with a treated surface. When inserted first into a treated cage, and subsequently allowed access to untreated cages, a higher mortality was recorded than when the procedure was reversed. The degrees of mortality and of restlessness appeared to be closely connected.