A 0.9‐mile section of Back Creek at Glengary, West Virginia, was sprayed by airplane with wettable DDT at a rate of 1 pound per acre. Analysis of the deposit showed that 0.39 pound of DDT per acre was actually laid down on the stream surface. Quantitative bottom samples taken of the insect fauna in the riffles, before and after spraying, showed a survival of about 30 percent at the lower stations. Samples of drifting insects collected before and after spraying showed an immediate heavy loss of caddisfly larvae and mayfly nymphs. Caddisfly larvae were killed first. The largest number of insects drifting downstream occurred within the first 3 hours after spraying. At the end of 9 hours the number of drifting insects was reduced almost to normal. The wettable‐DDT spray did not affect surface insects, such as water striders and whirligig beetles, to any important extent. Since it is known that streams can recover from catastrophic losses of bottom insects as a result of floods, it is believed that no permanent harm is caused by a single application of DDT such as would ordinarily be used in the control of forest insects. Although the sprayed section of the stream contained large numbers of bluntnose minnows, spotfin shiners, silverling minnows, and fallfish, these species were little affected by the DDT application. Only 61 native fishes were found dead. One‐half of these were smallmouth black bass, whereas most of the remainder were stoneroller and common suckers. Practically all the fish affected were hatched during the 1946 season. Live‐boxes stocked with warm‐water fishes were placed at intervals above, below, and within the sprayed section. Of 374 fish exposed to DDT in these boxes, 10.7 percent were lost from all causes, including DDT. Wettable DDT applied at 1 pound per acre apparently is not so toxic to fish and fish‐food organisms as the same amount of DDT applied in an oil spray.