Subgenus Microtus, agrestis species group sensu Zagorodnyuk (1990). Considered a member of subgenus Microtus by Pavlinov et al. (1995a), Gromov and Erbajeva (1995), and Meyer et al. (1996), but placed in subgenus Agricola by Zagorodnyuk (1990). Although once regarded as conspecific with North American M. pennsylvanicus on morphological grounds (Klimkiewicz, 1970), chomosomal differences indicate those similarities to represent convergence, not phylogenetic alliance (Vorontsov and Lyapunova, 1976; Zagorodnyuk, 1990). Phylogenetic inference based on cytochrome b sequences also widely separated M. agrestis from any North American Microtus (Conroy and Cook, 2000a), and allozymic analysis isolated it by large genetic distances from nine other Eurasian Microtus sampled (Mezhzherin et al., 1993).

Distribution in Portugal, Spain and France and morphometric discrimination from other Microtus and Chionomys in the region documented by Madureira (1983). Kooij et al. (1997) employed multivariate analyses to identify dentaries extracted from owl pellets as either M. agrestis or M. arvalis; Brunet-Lecomte et al. (1996) confidently discriminated the two species based on analysis of m1s; Kapischke (1992) recognized six distinct molar surface patterns in samples from Germany. Chromosomal data reviewed by Zima and Kral (1984a). Almaça (1993) assessed variation in pelage color, morphometrics, and dental traits among samples from N Portugal, Massif Central of France, and French lowlands and found little support for traditional subspecies but detected clinal variation between highland and lowland populations. Sequence analysis of mitochondrial DNA subdivided Eurasian M. agrestis into three allopatric clades, suggesting extended isolation in different glacial refugia (Jaarola and Searle, 2002): western (W and C Europe, spreading from the Carpathians); eastern (Lithuania to C Asia, originating from the S Urals or Caucasus); and southern (Portugal to Hungary, expanding from the Iberian Peninsula and perhaps representing a separate species).