The female of an ascidicolous copepod, Haplostoma eruca (Norman, 1869), is redescribed on the basis of specimens living in the intestine of Ciona intestinalis (Linnaeus) from the Clyde Estuary, Scotland. The redescription resolves some taxonomic problems involving H. eruca in the original and succeeding works. Detailed morphological features of H. eruca reported in this paper make it easy to distinguish this species from its 10 congeners of subgroup 1 of Haplostoma. The following features are thought to be species-specific: rostrum with two small ventral tubercles in addition to hairlike sensilla; antennule with conspicuously truncated distal margin; integument around single copulatory pore with many minute tubercle-like elements; and conical caudal ramus with two unequal armature elements on medial margin near apex.