Unidentified helminth larvae in the eyes of the North Sea whiting, Gadus merlangus L., were recorded in an unpublished survey of parasites of gadoid fish by Dr Z. Kabata of the Marine Laboratory, Aberdeen, in 1957. Since then these larvae have been observed frequently and during the first 6 months of 1964 an effort was made to identify them. They proved to be the plerocercoids ofa trypanorhynch cestode.

The plerocercoids are usually free in the aqueous or vitreous humour, but occasionally one is found attached to the lens. Very often both eyes of the same fish are infested, the largest number of larvae recorded from one eye being ten, from a fish taken in Shetland waters. The other eye of the same fish contained eight larvae.

In several plerocercoids examined the proboscides were evaginated, thus enabling detailed drawings to be made of the armature at a magnification of × 900. A number of specimens were fixed in Gilson's fixative; transverse and longitudinal sections were cut and stained with Mallory's triple stain; whole mounts were stained with borax carmine.

Early larval stages are completely undifferentiated; they develop, later, to the stage shown in Fig. 1. A plerocercoid with a fully developed scolex measures 3·5–8·0 mm long by 0·7–1·6 mm broad.

The larva is composed mainly of faintly staining tissue resembling the parenchyma of adult cestodes. The scolex is withdrawn into a cavity, the receptaculum scolecis, in the anterior half of the plerocercoid. At the anterior extremity is a deep invagination from which, in early larval stages, the receptaculum scolecis begins to develop.