Tyrosine-fed rats develop corneal disease which mimics that found in the human metabolic disorder, tyrosinosis. By electron and polarizing microscopy needle-shaped birefringent crystals are demonstrable in early corneal epithelial lesions of tyronsine-fed rats. The crystals appear as negative images by electron microscopy, their content apparently extracted by fixation and/or embedding fluids. Crystals may be arranged in sheaves or bundles and pass from one cell to another disrupting the continuity of membranes of both cells and nuclei. They are present in desquamating epithelial cells along corneal ulcers. Polarizing microscopy of whole mounts of diseased tissue shows that the crystals are limited to epithelial lesion areas. We hypothesize that the crystals are tyrosine and that crystal growth in cells initiates lesion and subsequent ulcer formation.