THE eleven families and nearly one hundred species of ceratioid anglerfish are distributed throughout the world's oceans below a depth of 500 m. The Ceratiidae, with two monotypic genera, Ceratias Kröyer and Cryptopsaras Gill, is one of four ceratioid families whose members exhibit a peculiar and unique mode of reproduction in which dwarfed males become permanently and parasitically attached to the body of a relatively gigantic female. Males of this family have large, forwardly directed eyes, apparently relying entirely on vision for their search and identification of a conspecific female. As in other ceratioid males, they are also equipped with a set of pincher-like denticles at the tips of their jaws for grasping and holding fast to a mate. Attachment is followed by fusion of epidermal tissues, and eventually by a uniting of the circulatory systems, so that the male, whose single function is to produce sperm, becomes dependent on the female for blood-transported nutriment, and the female becomes a kind of self-fertilising hermaphroditic host. Since its discovery 50 years ago, the story of sexual parasitism in ceratioid anglerfish has become a part of everyday scientific knowledge, yet no thoroughly satisfactory analysis of the known facts concerning this remarkable reproductive strategy has been made, in spite of the elegant work of Bertelsen1. This report describes sexual parasitism in surprisingly young females of C. couesi. Contrary to previous thought, it is now evident that parasitic attachment can take place at an extremely early age immediately following metamorphosis.