The copepod described in this paper has been found in Strangford Lough, Co. Down, an area which has already proved rich in ascidicolous copepods of the suborder Notodelphyoida (Gotto, 1952). The present species, however, belongs to the section Poecilostoma of the suborder Cyclopoida, many members of which are associated with marine animals of several different groups. The extremes of retrogressive specialization which characterize many of the notodelphyoids are not, on the whole, so apparent in these cyclopoids, their associative habits often being inferred from general structure rather than proved occurrence with any particular host. Thus a fair number of the types described have been found unattached in dredged material, the possible host being any one of several species taken in the same haul. Associations of an apparently loose nature are therefore sufficiently frequent to render the present form of particular interest since it has so far been found exclusively in the compound ascidian Trididemnum tenerum (Verrill), in which it is localized in the extensive cloacal cavities. Further search may of course reveal alternative hosts, since the family Lichomolgidae, to which it undoubtedly belongs, has a wide range of acceptable hosts. Indeed the genus Lichomolgus is known to occur in association with sponges, polychaets, nudibranchs, gasteropods, cephalopods, holothurians and ascidians—and one species, L. agilis (Leydig) has been recorded from a score of different nudibranchs.