Aside from M. acerbus (Jordan), which is apparently a breeding parasite of Tamias striatus (L.), a chipmunk, the holarctic genus Megabothris Jordan 1933 appears to be truly associated with the Microtinae, some of the species showing special affinities for particular species of these mice, others infesting several species, or even genera, within a geographical region.

The nearctic Megabothris asio (Baker) favours the common meadow vole, Microtus pennsylvanicus (Ord), and occurs throughout most of the range of that mouse. The writer recognizes two valid subspecies, M. asio asio, which is restricted to eastern North America, and M. asio megacolpus (Jordan), of western North America. The nominate race ranges from Labrador to Delaware. M. asio megacolpus (=M. asio orectus Jord.) ranges from western Quebec, western Ontario, and Michigan westward to the Coast Range, and from California north at least to the 60th parallel.