Taxonomic and nomenclatural confusions encountered in a study of the British species ofCystoseiraare discussed; a number of non‐British species, problems of whose synonymy have become involved with those of the British species, are also included.Fucus concalenatusL. is shown to be a synonym ofF. foeniculaceusL. The combinationCystosira nodicaulis(With.) M. Roberts replacesC. granulata(L.) Grev., as the nameC. granulatais illegitimate and its type does not belong to this speoies.Fucus selaginoidesL. is the earliest description ofCystoseira tamariscifolia(Huds.) Papenf., but the combinationC. selaginoidescannot here be used since Naccari had earlier employed the name to described another species. The identity of the type specimenof Fucus barbatus L. and other descriptions of this species are considered.Cystoseira fimbriata(Desf.) Bory is the correct name for the species often referred to asC. abrotanifoliaCAg. The species currently known asC. discors(L.) Ag. has no legitimate name.Cystoseira iisneoides(L.) M. Roberts is shown to be the correct name of the species known asC. concatenataAg.Cystoseira selaginoidesNacc,C. sauvageauanaHamel andC. hoppiisensu Valiante are briefly discussed; the last has no legitimate nam