The taxonomic status of three closely related species of the brown algal genus Elachista, E. tenuis Yamada, E. sargassicola Noda and E. nipponica Umezaki (Elachistaceae, Ectocarpales sensu lato) was reassessed on the basis of morphological observations including laboratory culture experiments and molecular phylogenetic analyses. These species share characteristic assimilatory filaments of constant width throughout their length except for slightly constricted bases and slightly tapering apices. Although E. sargassicola has been characterized by the presence of phaeophycean hairs, our culture experiments revealed that the presence or absence of such hairs is not a stable taxonomic feature in E. tenuis and E. sargassicola. Furthermore, molecular analyses showed that the rRNA ITS1 and ITS2 regions of E. tenuis and E. sargassicola were nearly identical and grouped them in a strongly supported monophyletic clade. Thus, E. sargassicola should be reduced to synonymy with E. tenuis. On the other hand, E. nipponica, which has been characterized by the production of cylindrical cells in a basal hemispherical cushion, formed cylindrical cells under experimental culture conditions, indicating that the production of such cells is a specific feature of E. nipponica. However, another reported feature of E. nipponica, the production of intercalary plurizoidangia in assimilatory filaments, was rejected, because reexamination of type material revealed that filaments bearing such reproductive structures are referable to Leptonematella fasciculata (Reinke) Silva. Molecular phylogenetic analyses support the distinction of E. tenuis and E. nipponica.