The families Opecoelidae, Lepocreadiidae and Enenteridae are differentiated. The Enenteridae is considered to consist of three subfamilies, the Enenterinae, the Cadenatellinae subfam. nov. and the Pseudolepidapedinae, whose constituent genera are listed. The diagnostic features of the opecoelid subfamilies Opecoelinae, Plagioporinae and Stenakrinae are presented. Plagioporus is restricted to freshwater forms, and Lebouria is considered a synonym of Peracreadium, following the transfer of L. idonea Nicoll, 1909, to the latter genus as P. idoneum comb. nov. Caudotestis is considered to belong to the Stenakrinae, Trematichtys to the Allocreadiidae, Spinoplagioporus to the Enenterinae and Pycnadenoides to the Plagioporinae. Macvicaria gen. nov., with the type-species M. [Plagioporus] alacris (Looss, 1901) and also containing M. [Distoma] soleae (Dujardin, 1845) (syn. Plagioporus varius) is erected for some of the marine forms previously allocated to Plagioporus. Neolebouria is briefly discussed and N. merretti sp. nov. from Bathytyphlops sewelli is described. Gaevskajatrema gen. nov. is erected for type-species G. [Peracreadium] perezi (Mathias, 1926) and G. [Plagioporus] lethrini (Nagaty, 1942). Podocotyle is discussed, especially in relation to P. angulata (Dujardin, 1845), which is reinstated as the type-species, P. atomon (Rud., 1802) and P. staffordi Miller, 1941, and P. abitionis McFarlane, 1936, is transferred to Pellamyzon. Tellervotrema gen. nov. is erected for the type-species T. armstrongi sp. nov. from Nezumia aequalis, which is described, and T. [Plagioporus] beringi (Mamaev, 1965). The genera Peracreadium, Cainocreadium, Hamacreadium, Bathycreadium and Helicometra are also briefly discussed. In addition, a key to the plagioporine genera occurring in European Atlantic waters is presented.
