In addition to sexual selection, selection resulting from social interactions in contexts other than mating can be a potent evolutionary force. Such social selection processes are facilitated whenever individual fitness varies as a result of any form of social interactions. The choice of social partners for communal care of young is such a situation in which interactants potentially experience fitness variance. In this study, we investigated the existence and impact of female social partner choice and the potential for social selection to occur in the cooperatively breeding wild house mouse, Mus domesticus. We analysed patterns of individual associations in groups of females, and compared the reproductive behaviour of females grouped with either a preferred or a nonpreferred social partner over an experimental life span of half a year, using spatial association as a measure of preference. We predicted low reproductive competition among preferred social partners and high competition, reflected in lower reproductive success, among nonpreferred. Our results showed that female house mice displayed nonrandom preferences, and that social partner choice yielded significant fitness benefits. Females in pairs with a preferred partner had a significantly higher probability to give birth and to establish an egalitarian, cooperative relationship, resulting in higher reproductive success than females in nonpreferred pairs. This suggests that interactions among females are subject to social selection processes, driving the evolution of female traits.