1. Simple trials of the effect on mosquitos of single applications of DDT to the internal surfaces of houses were initiated in British Guiana during the period 27th January to 20th April, 1945.

2. The solution used in the first two trials was 4·6 per cent. DDT (72 per cent. para para) in kerosene applied at the rate of about 2 quarts per 1,000 square feet (i.e., approximately 100 mg. of crude DDT per square foot).

3. An apparent reduction in the house population of Anopheles darlingi, the chief malaria vector, followed the treatment. Reduction exceeded 99·7 per cent. over eight weeks in the treated houses on one estate and 98·8 per cent. over six weeks on another.

4. In a third trial with a small variety of simple formulations, promising results were obtained with a 6 per cent. emulsion and a 2½ per cent. solution. But mosquito catches over a short period suggest that mud walls, thatch and whitewash may interfere to some extent with the action of DDT.

5. The cost of applications was of the order of one shilling and sixpence for each room (or small hut) of about 1,000 sq. feet internal wall and ceiling surface. This estimate does not include European supervision or the initial cost and depreciation of apparatus.