Usually included in M. rufocanus (G. M. Allen, 1940; Ellerman and Morrison-Scott, 1951; Gromov and Polyakov, 1977; Hinton, 1926a; also see references in Kaneko, 1990) but treated as a separate species by Corbet (1978c). In a study of red-backed voles from Russia, NE China, and Korea, Kaneko (1990) thoroughly documented morphological distinctions between M. rufocanus and M. regulus, considering the latter to be a Korean endemic and suggesting (p. 129) that "the true geographical demarcation line between the two species lies on the western and southern boundary of the Kaima Plateau, North Korea." Kaneko also discovered that adult and old regulus lack molar roots, a key character for separating Myodes from Eothenomys, and so transferred the Korean endemic to Eothenomys.
Subsequent mitochondrial and nuclear ribosomal DNA analyses conclusively demonstrate that M. regulus is distinct but phylogenetically close to M. rufocanus, not typical Eothenomys (Suzuki et al., 1999b, whose study included melanogaster, the type species of Eothenomys; Wakana et al., 1996). Wakana et al. (1996:22) noted that the "absence of rooting of the molars in the Korean vole … is a characteristic that may have developed within a short period of evolutional time in the Korean population." The G-banding pattern of regulus is "essentially identical" to that of M. rufocanus, both differing from M. glareolus, but Y-chromosome morphology implicates closer relationship between M. regulus and M. smithii (Iwasa et al., 1999a), a Japanese vole also having unrooted molars. As understated by Iwasa et al. (1999a:40), phylogenetic affinities among rufocanus, regulus, and smithii "are extremely complicated." To this complicated mix must be added M. shanseius (also rootless), which is so morphologically similar to M. regulus that the same specimens have been variously identified as one or the other. They differ only in length of tail, interorbital breadth, and M3 occlusal patterns; as currently known, their distributions are allopatric, perhaps separated by the Liao He River in Liaoning Prov. (Kaneko, 1992c). The genetic integrity of M. regulus relative to M. shanseius similarly warrants additional study.