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The monogeneans Pseudodiplorchis americanus and Neodiplorchis scaphiopodis have two pairs of rhabdomeric eyespots, each composed of a cup-shaped supportive cell and a sensory component. The supportive cell is characterized by concentric rows of plates which reflect light onto the rhabdomere. Similar eyespots, with a reflecting supportive cup, have been described in four other, phylogenetically-diverse, invertebrate groups, but amongst platyhelminths they are recorded only in the Polystomatidae. The structure conforms to a multilayer quarter wavelength reflector which is adapted to low light intensities. By contrast with most other endoparasites, the eyes of P. americanus and N. scaphiopodis do not degenerate after host invasion but persist throughout life. The hosts (desert toads, Scaphiopus species) are strictly nocturnal during their brief activity season and spend the rest of the year in hibernation burrows. Whilst a very sensitive photoreceptor is likely to be important in night-time larval invasion, its role in the biology of the adult parasite is uncertain.