We experimentally investigated effects of female immigrants on demography and social organization of prairie vole (Microtus ochrogaster) populations. We tested the hypothesis that increased immigrant pressure by unrelated adult females would decrease the survivorship of juveniles and indirectly decrease the proportion of communal groups in the populations. We established prairie vole populations in 8 enclosures (32 × 32 m). Four populations were supplemented with adult female immigrants, and the other 4 served as control populations in which no females were introduced. The results showed that adult female immigrants reduced the proportion of females and the proportion of juveniles in resident populations but did not change the social organization. We concluded that adult female immigrants did negatively affect resident populations. However, the direct numerical input and subsequent reproduction by immigrants would compensate for their negative effect on resident populations.