Sampling of emerging aquatic insects by cage-traps on a 24-hr basis showed the effects on the stream fauna of spraying forests with DDT at [Formula: see text] in June. Except for a few individuals that presumably emerged from unaffected pupae and nymphs, there was an interval of several weeks after spraying when no emergence occurred. From about the middle of August, however, large populations of very small insects, mainly chironomids, emerged in the spray year. The year after spraying, as compared with the spray year, there was an increased variety including some larger species.In following years there was further recovery by increase in larger forms through reproduction from residual small populations. The caddisflies recovered more slowly than other groups. After a single application of spray the fauna had practically recovered qualitatively in 2 or 3 years in some groups but usually not for 4 years or more in caddisflies. With a further similar spray applied within 3 years a reduction occurred equal in severity to that of the original spraying.From the standpoint of feeding of the young salmon, significant facts are: (a) the severe reduction in the bottom fauna of larvae and nymphs of all sizes in the first weeks after spraying; (b) the large populations of small larvae, mainly chironomids, developing in late summer of the spray year and so available to fry; (c) increasing numbers of larger insects the year after spraying and subsequent years.