The objective of the proposed research is to develop a sufficient understanding of the snail-trematode and intertrematode interactions to be able to utilize this approach as a means for developing biological control methods. Emphasis will be given to the study of acquired and natural resistance to trematode infections in snails, and of indirect antagonism between two trematode species. With the use of appropriate snail-trematode systems, appropriate combinations of irradiated and nonirradiated trematode species, controlled dosage and infection intervals, along with standard histological and electron microscopic techniques, we shall study the underlying principles of acquired and natural resistance and their possible relationship with indirect antagonism. This includes the study of the amebocyte-producing organ; the investigation of whether humoral factors play a role in the snail's defense; the study of specificity of acquired and natural resistance; and the study of the interference capacity of trematode larvae. BIBLIOGRAPHIC REFERENCES: Lie, K.J., and D. Heyneman 1976. Studies on resistance in snails. 3. Tissue reactions to Echinostoma lindoense sporocysts in sensitized and resensitized Biomphalaria glabrata. J. Parasitol. 62: 51-58. Lie, K.J. and D. Heyneman and K.H. Jeong 1976. Studies on resistance in snails. 4. Induction of ventricular capsules and changes in the amebocyte-producing organ during sensitization of Biomphalaria glabrata. J. Parasitol. 62: 286-291.