Differences in the social behavior of monogamous and polygamous rodents may be related to estrogen receptor subtypes (ERs). The distribution of estrogen receptor α (ERα) and β (ERβ) immunoreactivity in polygamous rats and mice, and ERα immunoreactivity in the central nervous system of monogamous male prairie voles, have been reported, but no data on the ERβ distribution in monogamous voles is available. In the present study, the numbers of ERα and ERβ immunoreactive (IR) cells in the forebrain of male and female mandarin voles (Microtus mandarinus) were compared using immunocytochemistry. We found that ERα-IR and ERβ-IR cells were mainly found in nuclei in mandarin voles. ERα-IR and ERβ-IR cells were widely distributed throughout the forebrain and expression overlaps in many forebrain areas. However, the number of cells and degree of receptor expression per cell were different for each ER subtype. In the medial preoptic nucleus (MPN), bed nucleus of the stria terminalis (BNST) and medial amygdaloid nucleus (MeA) females had a higher ERα density than did males. The opposite pattern was found in the MPN and BNST where males had a higher ERβ density than did females. These results provide an important neuroanatomical basis for enhancing our understanding of how estrogens work through ERα and ERβ to regulate important processes including social behavior.