Plymouth scad (horse-mackerel) Trachurus trachurus (L.) are known to harbour three species of monogenean trematode gill parasites, of which two belong to the Gastrocotylidae and one to the Microcotylidae. The oncomiracidia (= newly hatched larvae) of these parasites have already been described (Llewellyn, 1957a), but nothing is known of the developmental stages intervening between the oncomiracidia and the adults. Bychowsky (1957) has stated that some information is available about the larval development of 13 monogenean families, leaving 15 families, including the Gastrocotylidae, about whose larval development nothing is known. The ontogenetic development of the Gastrocotylidae is of especial interest since the oncomiracidia are bilaterally symmetrical, but the adults invariably show an extreme degree of asymmetry.