Described as a species of Clethrionomys, imaizumii was demoted to a subspecies of andersoni by Aimi (1967) and afterwards placed in full synonymy of Eothenomys andersoni (Aimi, 1980; Musser and Carleton, 1993; Pavlinov et al., 1995a). See M. smithii account for association of imaizumii with Myodes rather than Eothenomys. Kitahara (1995) exhaustively studied molar patterns and craniometric data of C Honshu voles and also concluded that the Kii populations represent M. andersoni; lab-breeding trials between imaizumii and andersoni have produced fertile hybrids (Kitahara and Kimura, 1995). Suzuki et al. (1999b) postulated a biogeographic scenario to explain the southward extension of andersoni into the Kii Peninsula, followed by their isolation, as imaizumii, when climates warmed and M. smithii expanded from west to the east.
Detailed chromosomal (Iwasa et al., 1999b) and molecular studies (Suzuki et al., 1999b), however, just as convincingly attest the absence of genetic interchange between andersoni and imaizumii. As noted by Iwasa et al. (1999), imaizumii is now geographically widely isolated from andersoni and their ability to produce fertile hybrids in the wild is untestable. MtDNA sequences portray imaizumii as unique, but its rDNA profile reflects a mixture of those characterizing M. andersoni and M. smithii, leading Suzuki et al. (1999b:519) to wonder whether "ancestral populations of smithii andersoni were polymorphic at each of the variable restriction sites and fixation of variants progressed during speciation, except for E. imaizumii in which the polymorphic status has been preserved until now." Pending completion of such on-going studies, we follow Kaneko (1994) in retaining imaizumii as species based on its distinctive molecular traits and isolated geographic range.