A first experiment on malaria control in the interior of Borneo by spraying with residual insecticides is described. The work was carried out in the isolated, sparsely populated valleys of the Baram River and its tributary, the Tinjar, in northern Sarawak. The experimental area was divided into three parts: a DDT test area, where a 75% suspension of wettable powder was applied at the rate of 2 g of DDT per m(2) of surface; a BHC test area, where a 50% suspension of wettable powder was applied at the rate of 0.10 g of gamma isomer per m(2); and a check area.Entomological investigations made before the spraying operations were started showed that Anopheles leucosphyrus Dönitz, 1901 was the main malaria vector in both the test and the check areas. Out of a total of 7568 A. leucosphyrus dissected, 30 gland infections were detected-a sporozoite-rate of 0.40%. A. barbirostris was found to be a secondary vector throughout the experimental area.THE RESULTS OF INSECTICIDE SPRAYING WERE SATISFACTORY: in the DDT test area, the spleen-rate fell from 51.8% to 25.1%, and the parasite-rate from 35.6% to 1.6%, in 21 months, and a similar reduction in the rates was observed in the BHC test area. In the check area, the spleen- and parasite-rates rose during the period of observations. It is considered that if such a degree of control can be obtained in 21 months, complete eradication can be expected in the near future.Although BHC spraying proved effective, the fact that it has to be repeated every three months makes it impracticable in the interior of Sarawak, where communications are very poor and difficulties of transport very great. DDT spraying, which need only be done twice a year, is therefore to be preferred. The cost of the DDT operations-US$ 0.45 per person protected per year-is comparatively high, owing to the difficulty of communications and to the necessity for spraying not only the village "longhouses", but also the temporary shelters which the semi-nomadic people in the interior of Sarawak build each year in the rice-fields.