A study of the possible cumulative effects of DDT on arthropods in streams treated for black fly larval control for 5 to 10 years was carried out in 1961. Statistical analysis based on 348 square-foot riffle samples taken in treated and untreated Adirondack Mountain streams indicates that a significantly smaller number of Ephemeroptera and Diptera were present in the regularly treated streams than in those untreated, and that the reduction in overall numbers of arthropods approached hut did not reach a significant level. The numbers of arthropods in 11 regularly treated streams sampled in 1950-52 and again in 1961 were about the same during the two sampling periods.

The ratio of weight to volume to numbers of arthropods was about the same in regularly treated, irregularly treated, and untreated streams sampled in 1961.