Field experiments have been made with DDT in oil as a larvicide on flowing water against Anopheles maculatus. 1. In unweeded drains and streams, 2 oz. DDT per acre applied evenly with a pipette as a 5% solution in Malariol, gave control 1 day later of over 900%. 2. The same dose in an oil (Malariol H S) with a very high spreading pressure (33 dynes/cm) when applied at intervals of 30 yards gave only 50% control. When applied evenly control was 98%. When applied at intervals of 22 yards (one chain) in a grassy roadside drain, control was 61% before the drain was weeded and 73% afterwards. The DDT in oil when applied at intervals was prevented from reaching all the larva e by strong surface films, obstructions and side pockets, and it is clear that to control A. maculatus application must be continuous and not at spaced intervals. 3. Continuous application by spraying with a Mish pump was compared with even distribution of the same dose with a pipette. Control by pipette was only 72% on this occasion, due probably to a heavy growth of weeds, but control by spraying was considerably less, only 56%, apparently because much of the fine spray was blown away before reaching the water. The sprayer was very tiring to use. 4. DDT in Malariol with a spreading pressure of about 20 dynes/cm was compared with DDT in Malariol H S (S P about 33 dynes/cm). Both solutions were applied at intervals of 30 yards at the rate of about 2 oz. of DDT per acre. Control 1 day later was 84% with DDT in Malariol H S and 78% with DDT in Malariol. It is concluded that under the adverse conditions of these experiments (drains not weeded or maintain ed, DDT applied at intervals), better control is obtained by using oil with a very high spreading pressure. 5. Doses of about 4 ounces and 2 ounces of DDT per acre were compared. Application was by even distribution with dropping bottles. The heavier dose gave somewhat better immediate control, and had a better lasting effect, delaying the reappearance of large larvae (4th instar), usually until later than the sixth day after treatment. 6. DDT in Malariol applied evenly with dropping bottle at about 4 oz. DDT per acre was compared with ordinary oiling with a knapsack sprayer at about 23 gallons per acre. The experiment was made in weeded and well maintained drains in the Kuala Lumpur oiling area. Both treatments gave complete immediate control, no larvae being found two days afterwards, but breeding recovered a little more rapidly after the DDT treatment, though there were no large larvae on the sixth day. 7. Numerous readings were made of the strength of natural surface films on the water of breeding places. The results show a fair measure of agreement with those obtained in West Africa by Toms. The commonest film strength was found to be 7.5 – 13.0 dynes/cm., but the proportion stronger than this varied from place to place. The readings from the breeding places of A maculatus suggest that for an ordinary anti malarial oil a spreading pressure close to 25 dynes/cm will usually be sufficient. 8. It is concluded that DDT in oil at about 4 oz. of DDT per acre (=½gal of a 5% solution) can give satisfactory control of A maculatus, especially in properly maintained drains. However, the solution must be applied evenly, and with such small quantities there seems as yet to be no method of doing so which would be suitable for general use. Possible ways of overcoming this difficulty are discussed.