A relatively small number of Marisa cornuarietis was introduced into a chain of five ponds containing well established Australorbis glabratus populations. During an 81-week observation period the Marisa effectively reduced the Australorbis population almost to extinction. The population has not recovered in one and a half years. Additional studies are indicated to determine if Marisa will work equally well in other Australorbis habitats.
Present address: Department of Medical Zoology, Walter Reed Army Institute of Research, Washington 12, D. C.
Medical Zoology Division, U. S. Army Tropical Research Medical Laboratory, APO 851, New York, New York.
Puerto Rico Field Station, Technical Development Laboratories, Technology Branch, Communicable Disease Center, Public Health Service, Department of Health, Education, and Welfare, San Juan, Puerto Rico.