Most West-Palearctic Mesoleius auct. have been described by Swedish authors, whose collections are preserved in the Museum of Natural History, Stockholm (Holmgren and Roman) and in the Entomological Museum, Zoology Department, Lund University (Thomson). Because the collections are easily found and are kept in two museums only it was possible to make a preliminary revision of these insects, noting for each species the main structural characters in accordance with the keys of H. Townes (Genera of Ichneumonidae, Part 3, 1970). Ovipositors were removed from their sheaths and described. It appears that eight genera were mixed with the Mesoleius of Schmiedeknecht. The present paper attempts to place each species in its true genus and to critically examine the characters used by Townes. I believe that most European species of Schmiedeknecht's Mesoleius should be transfered to the genus Campodorus Förster. Species of the genus Anoncus Townes are easily recognized by their tarsi tending to be short and by their filiform ovipositor. Species of Hyperbatus Förster have the extremity of the abdomen flattened, have short square sheaths, and a very brief apex after the dorsal notch of the ovipositor. 161 species of the genus Mesoleius sensu Schmiedeknecht 1914, and two additional Mesoleius sensu Hinz 1972, were studied. They were found to belong to eight genera: Mesoleius Holmgren, 1855 (45 species), Campodorus Förster, 1868 (86 species of which 78 are new combinations), Anoncus Townes, 1970 (15 species all of which are new combinations), Otlophorus Förster, 1868 (2 species one of which is a new combination), Alexeter Förster, 1868 (5 species all of which are new combinations), Scopesis Förster, 1868 (2 species), Hyperbatus Förster, 1868 (5 species of which 3 are new combinations) and Barytarbes Förster, 1868 (1 species which is a new combination).