Cercariae of Diplostomum spathaceum dropped onto the eyes of small unanaesthetized rabbits penetrated through the cornea, crossed the anterior chamber, and entered the lens. After 2-3 weeks they died and became semi-permanent amorphous cataracts. Cercariae remaining in the cornea became the centres of stromal nebulae 0.1-0.2 mm across which remained visible for at least 3 months. Re-exposure produced marked conjunctival inflammation. Parasites applied to the eyes of large adult rabbits or cold-stored enucleated eyes of man entered the cornea but failed to penetrate as far as the anterior chamber. It is suggested that cercariae are unlikely to reach the lens of bathers because of the thickness of the human cornea. However, exposure to infection may result in temporary conjunctival inflammation and more persistant stromal nebulae. Cercariae of the closely related D. adamsi, did not penetrate or develop in the eyes of mammals.