Morphological characteristics of the terrestrial (Acteonemertes, Antiponemertes, Argonemertes, Katechonemertes, Leptonemertes) and freshwater (Campbellonemertes, Potamonemertes) paramonostiliferous nemerteans, all of which are at present unplaced at the family level, are compared and contrasted with those of the marine genera Plectonemertes (Plectonemertidae), Cratene-mertes, Korolkevilschia and Nippormemertes (Cratenemertidae). It is concluded that whilst Acteonemertes, Antiponemertes, Argonemertes, Campbellonemertes and Potamonemertes can with certainty be placed in the family Plectonemertidae, the unspecialized and geographically isolated genera Katechonemertes and Leptonemertes, although lacking positive grounds for their exclusion, are only questionably included in this taxon. The possible evolutionary relationships of the plectonemertid genera are discussed and a diagnosis of the family is provided.