Fortunate circumstances have recently allowed to collect specimens of the late Eifelian Isopoma brachyptyctum (SCHNUR 1853) from its type locality (Blankenheim, Blankenheimer Mulde, Eifel area). For the first time internal characters were investigated by means of transverse serial sections of topotypic material. Apart from its original description, this species, designated as the type species of the genus Isopoma TORLEY 1934, was only known so far by its lectotype. A full description of Isopoma is given, and its original description by TORLEY, based on late Givetian specimens from Sauerland unduly identified as I. brachyptyctum, is disregarded. A new family, Isopomidae, type genus Isopoma, is established. A better understanding of Isopoma made it possible to assess the validity of the various species and subspecies assigned to the genus at one time or another.