The introduction of immigrants into resident populations may disturb the social organisation of the latter. It is often stated that males compete with males for access to mates, while females compete with females and/or males for limited resources (e.g., nest sites, food). Therefore, the impacts of introductions on residents should depend on the immigrant's sex. To test this hypothesis, we experimentally introduced either male or female immigrants into field vole (Microtus agrestis) populations, and observed the consequences in terms of space use and survival of both residents and immigrants. Our results showed intra-sexual competition: (i) resident females responded to the introduction of females by reducing their home range size and their interactions with neighbours; (ii) immigrant males suffered high mortality, probably due to contests with resident males for access to females. We conclude that the role of social interactions should not be underestimated when releasing unfamiliar individuals into small populations.