The female of Haplostoma brevicauda (Canu, 1886) is redescribed primarily based on specimens living in the Ascidian Aplidium argus which is the type host. Three other Aplidium species have been confirmed to be parasitized by H. brevicauda. The redescription indicates that H. mizoulei Monniot, 1962, is synonymous with H. brevicauda. Subgroup 1 of Haplostoma, which consists of H. brevicauda and 10 congeners, exhibits at least four patterns (I–IV) of armature formulas for legs 1–4; H. brevicauda has pattern II. In this paper, the male of H. brevicauda is described for the first time based on specimens living in A. turbinatum and one other Aplidium species. A male that Canu (1892) reported as the male of H. brevicauda will be assigned here as the male of Botryllophilus ruber Hesse, 1864. The present material was obtained by the author at Roscoff, France, in 1992.