The migratory route of Diplostomum spathaceum to the eye lens of the fish host was determined by examining fishes killed at various times after infection in the laboratory by immersion in water containing cercariae. Migration occurred mostly in subcutaneous connective tissue and muscles of the trunk. Neither peritoneal cavity nor circulatory system served as a major migratory route. Rate of migration was estimated to be about 5 mm/hr with the aid of caudal fin immersion technique. Successful migration depended largely on adequate glycogen reserves and secretions of penetration glands. Only diplostomula that could plow their way to fish lenses survived the host tissue reactions; the remainder underwent phagocytosis.