The larval form (late cercaria stage) of Pharyngora bacillaris (Molin) has been recorded by Nicoll (1910) from Plymouth as occurring free in the coarse-meshed tow-nettings in August. The adult is a common parasite of the mackerel, and the above is the only record of its larval stage.Whilst examining tow-nettings at Plymouth in 1915 it was found that certain medusæ were at times abundantly infected with a trematode which proved to be the larval form of Pharyngora bacillaris. As it also occurred free, although almost certainly having originally come from the medusæ, it is obviously the form recorded by Nicoll, who expected the host to be a crustacean and unsuccessfully examined copepods in order to find it.The medusæ found to contain the trematode were Obelia sp., Cosmetira pilosella, and Turris pileata. Cosmetira pilosella was the commonest host in the early summer when Pharyngora was most abundant, but in the later summer Obelia was found to contain it frequently, Cosmetira not occurring at those times in the tow-nettings. Phialidium hemisphericum was also a host in the later summer and autumn. Even in December it still occurred, though very rarely. A ctenophore may also serve as host for this trematode, as it is occasionally found clinging to the inside of the stomach of Pleurobrachia pileus.The parasite is generally to be found clinging to the manubrium or stomach wall of its host, but sometimes it occurs underneath the umbrella wall, so that it looks as if it were on the top, the wall being so transparent; on further examination, however, it is seen to be underneath.