A description is given of the morphological and ultrastructural characteristics of the eye of Mordacia mordax, one of the two parasitic species of the southern hemisphere lamprey family Mordaciidae. The avascular retina is well-differentiated, containing two ganglion cell sublaminae, two layers of horizontal cells and a large population of displaced bipolar cells. Intermediate filaments, which stain positively for glial fibrillary acidic protein (GFAP), lie in perpendicularly-arranged bundles between the two layers of horizontal cells. M. mordax contains only a single type of rod-like photoreceptor, in contrast to representatives in the other two lamprey families Geotriidae and Petromyzontidae, which possess three (two cone-like and one rod-like) and two (one cone and one rod) types of photoreceptor, respectively. The large photoreceptors of M. mordax possess a large mitochondrial ellipsosome, a cylindrical outer segment with incisures, and a nucleus immediately abutting the outer limiting membrane. The photoreceptors are arranged in an hexagonal array with their outer segments surrounded by a retinal tapetum which elicits a yellow fundal eyeshine. The tapetum contains both diffuse (vitread spheroidal granules) and specular (sclerad needles) reflectors. The characteristics of the eye of M. mordax are related to the diurnal burrowing of this species during its upstream migration, a feature not found in the other two extant families of lampreys. The absence of cone-like photoreceptors and the presence of a tapetum suggest that although photopic vision is probably greatly diminished, characteristics have been evolved by the eye to maximize light capture and increase sensitivity in the low light intensities encountered during this species' nocturnal migration.