Thiara granifera is a melaniid snail capable of maintaining very high densities in a variety of habitats. It has been introduced into the New World from the Far East and is now spreading rapidly throughout the Caribbean. In Puerto Rico and Dominica casual observations following natural invasion by T. granifera suggest that it may exert a powerful restraining influence on populations of Biomphalaria glabrata, the major intermediate host of Schistosoma mansoni in the Caribbean. The potential of T. granifera in biological control is being investigated in St. Lucia. In four field trials, B. glabrata was apparently eliminated from marshes and streams six to 22 months after the introduction of T. granifera. Thiara granifera shows promise as a major factor in the suppression of schistosomiasis in the Caribbean, but it is unsuitable for universal use as it is an intermediate host of the lung fluke, Paragonimus westermani.