Attack by the cercariae of Schistosoma haematobium, S. mansoni and S. japonicum causes in man debilitating diseases which in certain countries are widespread. Infection is mainly through the skin, though it can occur through drinking water containing the parasites. It has been shown by Sproule (1939) that freshly liberated vigorous cercariae of S. haematobium and S. mansoni are killed in 30 min. by superchlorination to the extent of one dose beyond that indicated in the Horrocks test. This of course does not deal with skin infection, and though bathing in fresh water may be avoided and water drawn off may possibly be stored till all cercariae are dead, yet it is obviously impossible entirely to avoid contact with natural waters. In U.S.A. and Canada dermatitis is caused at many otherwise desirable lacustrine bathing beaches by cercariae liberated from Stagnicola emarginata and other snails which live in shallow water on sandy bottoms (Cort, 1928; Brackett, 1941). The final hosts are apparently water birds and in man these cercariae tunnel in the epithelial layer of the skin and soon disintegrate. No dangerous disease is caused, but there is much discomfort, especially among children who naturally keep to the shallow water. Penetration takes place largely as the water film dries and oiling the skin is no preventive. McMullen (1941) has given an account of attempts to eliminate the parasites by poisoning the snail hosts with copper sulphate or a sulphate carbonate mixture. A temporary success can be achieved at considerable cost.