Mesotrophic quaking fen hollows in the Dutch nature reserve ‘Het Hoi’ (near Kortenhoef in the Province of Noord-Holland) harbour a rich desmid flora. Some rare, taxonomically problematic species are discussed separately. Pleurotaenium coronatum var. robustum and P. nodulosum, in former days mutually confused, are compared in their differentiating characteristics. Closterium didymotocum var. crassum is recombined to C. baillyanum var. crassum, principally based on the shape of its cell apex. It cannot be totally excluded, however, that the taxon in question has to be considered a mere anomalous form of C. baillyanum var. baillyanum. A Closterium-species that could not be identified in a satisfying way shows much resemblence with C. macilentum, except that girdle bands are lacking.