Experiments were conducted in 200-gal concrete tanks to evaluate a large fresh-water snail, Marisa cornuarietis, as a biological aquatic weed control agent. Adult Marisa snails were collected from a canal near Miami, Florida, where a colony had recently become established. These snails are of South American origin and appear identical to those being used in Puerto Rico for biological control of schistosomiasis. The snails controlled Ceratophyllum demersum, Najas guadalupensis, and Potamogeton illinoensis completely and Pistia stratiotes and Alternanthera philoxeroides partially. Eichhornia crassipes was not completely eaten, but its growth and flowering were greatly retarded by the root-pruning action of the snails. Marisa preferred submersed weeds to floating or emersed weeds, but the floating weed Salvinia rotundifolia was eaten nearly as readily as submersed weeds. Little damage was done by Marisa to 4- and 5-week-old rice plants, but younger rice was killed when the snails had no other source of food. A common snail-eating bird, Cassidix mexicanus, was found to be a predator of Marisa in Florida. Except for its possible deletarious effects in rice-growing areas, Marisa was regarded as very promising for control of aquatic weeds at least in confined bodies of water.