Eleven chemical compounds which proved effective in killing A. glabratus in the laboratory in dilutions of 10 ppm. or less were tested on this same snail in its natural environment in Puerto Rico. Six of these proved to be very effective, although the present price of 4 of them (2,4,6-triiodophenol, 2,4,6-triiodo-phenol sodium salt, 2,4,6-tribromophenol, and pentabromophenol) may prohibit their use on a large scale. Two compounds (sodium pentachlorophenate and copper pentachlorophenate) are excellent molluscicides and their cost is reasonable. In a stream near Los Pena, sodium pentachlorophenate at 9.5 ppm. calculated on a 6-hr. flow rate dose destroyed all snails for a distance of 1.5 miles downstream in spite of the entrance of 3 untreated tributary streams within this area. Embryos within the snail eggs were also killed. The compound was lethal to catfish, guppies, and eels, but apparently did not affect crayfish. The toxicity of the effective compounds for mammals is under further study, and field trials are being extended to other areas.