This is part 12 of a series of monographs on the ichneumon-flies of America north of Mexico. The Microleptinae of the Nearctic Region are represented by three genera and four species; the Helictinae by 17 genera and 104 species; the Cylloceriinae by two genera and nine species; and the Oxytorinae by one genus of three species. A total of 29,500 specimens from the Nearctic Region were studied for this revision. An additional 1232 exotic specimens of species occurring in the Nearctic Region were examined.