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The oncomiracidium of the monogenean skin parasite Entobdella soleae finds its flatfish host, Solea solea, by chemoreception. The free-swimming larva responds to a specific substance secreted by the skin of the common sole, attaches itself to this skin and immediately sheds its ciliated epidermal cells. Larvae respond in the same way to agar jelly which has been in contact with the skin of S. solea.The oncomiracidia attach to S. solea skin in preference to that of other soleid fishes (Buglossidium luteum and Solea variegata), pleuronectid fishes (Limanda limanda, Pleuronectes platessa) and elasmobranch flatfishes (Raia spp.).Larvae respond strongly to isolated epidermis from Solea solea but show no response to the fish's cornea, indicating that the attractive substance is produced by the mucus cells in the fish's epidermis.The larvae attach with equal readiness to skin from the upper and lower surfaces of S. solea. Thus the preponderance of young parasites on the upper surfaces of soles is due not to a preference for the upper skin but to the fact that the lower skin is in contact with the substratum and cannot be reached by the larvae.These results led to speculations on the way in which host specificity evolved in the Monogenea.I am indebted to Mr J. E. Green of the Plymouth Laboratory for setting up a tank containing infected soles and for maintaining the tank and feeding the fishes for many months.I am also grateful to the Directors and Staff of the Plymouth Laboratory and the Fisheries Laboratory, Lowestoft, for hospitality and assistance. I am particularly grateful to Mr J. Riley of the Lowestoft Laboratory for providing various flatfishes.