A condition of algal nuisance in small ponds involving the water net, Hydrodictyon reticulatum, is described. A serial treatment with copper sulphate, in which daily applications of 0.33 p.p.m. of the chemical are made, resulted in the complete removal of the algae in five days. Sixteen ponds are kept free of algae by serial treatments with copper sulphate at regular intervals. The advantages of serial treatment over one-dose-treatment are discussed.