In tow-nettings from Plymouth Sound, June 20th, 1935, Acartia clausi were abundant and inside these copepods, emerging from them and free in the water were very large numbers of larval trematodes, easily identified as Hemiurus communis. This is not a new record, for this parasite has already been described from the same Copepod in Plymouth waters in May and February (Lebour, 1923), but it has always been rare. In no case have more than two parasitised specimens been seen from these regions, and no seasonal appearance has been noticed. Much has been written about these trematodes which grow in the bodies of copepods until they are of such a large size that they inevitably kill the host and just before it dies emerge from between the joints, if not eaten before this with the copepod, but I think this is the first time that they have been recorded in any quantity at one time. Since my first communication Steuer (1928) has written about the geographical distribution of these hemiurids parasitising copepods and is of the opinion that all those in Northern seas belong to Hemiurus lühei and H. communis. In the Mediterranean the hemiurids known have not yet been found in copepods but usually in Sagitta, or are found living freely in the sea and these probably belong to H. rugosus which lives in the adult state in the pilchard.