Records of the neotropical tropicorbid snails from the United States are from Florida near Miami, several localities in southeastern Louisiana and from Texas. Colonies derived from snails taken from lakes on the Louisiana State University campus in Baton Rouge were demonstrated by several workers to be susceptible to infection with Schistosoma mansoni. It is the objective of this research to conduct a survey in Louisiana and Texas to determine the distribution of tropicorbid snails, to study their systematics, to test their susceptibility to infection with Schistosoma mansoni, and thus to reveal the foci of potential schistosomiasis in this part of the United States. Possible mechanism(s) of resistance in certain populations, against S. mansoni are being investigated by electrophoretic studies. The research will also be extended to determine the distribution, bionomics, systematics and host parasite relationships of lymnaeid snails in the same geographical area. These are hosts of the mammalian schistosome of public health significance, Heterobilharzia americana and of Fasciola hepatica.