Interspecific affinities unclear: affiliated with Microtus sensu stricto, in particular M. californicus (Anderson, 1959, 1960; Conroy and Cook, 2000a; Conroy et al., 2001) or M. montanus and M. pennsylvanicus (Moore and Janecek, 1990); associated with the subgenus Pitymys by Hooper and Hart (1962); chromosomal banding data uninformative (Modi, 1987); also see Hoffmann and Koeppl (1985). Morphometric, electrophoretic, and karyotypic variation among isolated populations in SW USA studied by Wilhelm (1982); nongeographic variation of US and Mexican populations evaluated by Frey and Moore (1990); geographic variation of populations in C México assessed by Alvarez and Hernández-Chávez (1993), who considered the regional subspecies as valid (fundatus, mexicanus, and salvus) and described a new one (ocotensis).

Critical overhaul of the mexicanus complex is needed. Musser (1964) reduced the Oaxacan form fulviventer to a subspecies of M. mexicanus; whereas, Frey and LaRue (1993) and Frey (1999) elevated populations in the SW USA (mogollonensis) as distinct, arguing the truly superficial basis for its original synonymy (Bailey, 1932) and noting earlier reports of slight chromosomal differences between US and Mexican populations (Judd, 1980). Fitzgerald et al. (1994) and Saldaña-DeLeon and Jones (1998), on the other hand, retained mogollonensis as a subspecies for Colorado populations; furthermore, the interpretation of extensive and relatively recent post-Pleistocene dispersion of M. mexicanus into the SW USA (Davis and Callahan, 1992) questions the length of isolation of these populations and the potential for their differentiation as a separate species. A phylogeographic approach, integrating morphological and genetic variation across the collective range of these taxa, would offer a firmer basis for delineating species limits and diagnosing them.