Three chemicals, sodium pentachlorophenate, dinitro-o-cyclohexylphenol, and its dicyclohexylamine salt, have been tried under field conditions in the control of Oncomelania nosophora. At the dosages used they gave similar results but the first two were much less expensive. These two chemicals cost about one half as much as calcium cyanamide, a compound that has been used in snail control in the area where these experiments were conducted. The more than 18 miles of ditches treated offered a wide variety of habitats. The molluscacides were least effective in areas where the sides of the ditches had loose rock walls. The reduction in the snail population averaged 81.7 per cent in areas treated in October. When treated in the spring the average reduction was 87.7 per cent. When the molluscacides were applied in the autumn and the spring the reduction averaged 95.4 per cent. Surveys conducted the following autumn showed that a few young snails were present in some of the areas. Eradication of Schistosoma japonicum in an area would necessitate a more or less continuous surveying and use of a molluscacide, until the snails were eliminated, or the human and reservoir hosts were no longer infected.
406th Medical General Laboratory, Tokyo, Japan; and the University of Oklahoma School of Medicine.