The fauna of harpacticoid copepods associated with a North Atlantic sublittoral population of Laminaria ochroleuca in Spain was investigated. Copepods belonging to four different orders, i.e. Harpacticoida, Cyclopoida, Poecilostomatoida and Siphonostomatoida, most of which were harpacticoids belonging to 17 different families were collected. Because the family Thalestridae was particularly abundant in the samples, its members were identified down to the species level, totalling 20 species. The copepod fauna was richer and more diverse on holdfasts than on fronds, irrespective of considering the entire harpacticoid assemblage at the family level or just the family Thalestridae at the species level. The effect of these two within-plant microhabitats on the taxonomic composition of the copepod fauna was substantial, with faunal similarity between fronds of plants from different sampling quadrats being usually higher than that between the frond and the holdfast of the same plant. Thalestridae, Harpacticidae and Tisbidae predominated on the fronds, while Diosaccidae, Ameiridae and Ectinosomatidae predominated on the holdfasts. Among the thalestrids, Parathalestris clausi, Paradactylopodia sp.1, Dactylopusia tisboides, and Dactylopusia vulgaris were common species, with Parathalestris clausi being the most abundant copepod on both fronds and holdfasts. In summary, the fauna of the holdfasts consists mostly of common, eurytopic species, while the fauna of the fronds comprises rare taxa—at least within the scope of the study—with low total abundances and/or usually occurring in few plants.