The North and Central American species formerly placed in the genus Mycetoporus are revised. Two genera are recognized, Mycetoporus Mannerheim and Ischnosoma Stephens, each with 18 species. The morphological and historical basis for this division is discussed. Ten species of Mycetoporus are described as new: bipunctatu, floridensis, impunctatus, neomexicanus, nidicola, pacificus, rufohumeralis, segregatus, smetanai, and triangulatus. Ten species of Ischnosoma are described as new: arizonense, ashei, costale, durangoense, fimbriatum, hermani, lecontei, mexicanum, pecki, and suteri. Mycetoporus boreelus J. Sahlberg, a species restricted to the Palearctic region, is removed from synonymy with M. nigrans Mäklin and Mycetoporus insignis Mäklin is found to be a junior synonym of M. americanus Erichson. Mycetoporus discalis, M. punctatissimus, and M. punctulatus, all species described by Hatch (1957), are transferred to the genus Bryoporus subgenus Bryophacis and the name Tachinus humidus Say is treated as a nomen dubium in the genus Mycetoporus.The usage of the generic names Mycetoporus Mannerheim and Ischnosoma Stephens is discussed. The North American species of the genus Mycetoporus are placed in six species groups and those of the genus Ischnosoma are placed in four species groups.A neotype is designated for Mycetoporus nigrans Mäklin and lectotypes are designated for M. americanus Erichson, M. insignis Mäklin, M. tenuis Horn, M. lucidulus LeConte, M. consors LeConte, Ischnosoma pictum (Horn) and I. flavicolle (LeConte). The following species were transferred from Mycetoporus to Ischnosoma and are new combinations: pictum Horn, splendidum Gravenhorst, flavicolle LeConte, virginiense Bernhauer, coxale Sharp, hospitale Fall, californicum Bernhauer and Schubert, and curtipenne Bernhauer.All genera, species groups, and species are described and the species are illustrated with line drawings and scanning electron photomicrographs. Keys are provided to distinguish the genera Mycetoporus and Ischnosoma and for all the species in each genus. The New World distribution of each species is mapped. The biology of each species, if known, is discussed.
