Societies in which women have substantial control of resources and hold powerful political positions are relatively rare. Among the many circumstances in which women are likely to have resource control and/or political authority, polygyny is not an obvious candidate. However, women's lives are highly variable across polygynous societies. We hypothesized that within polygynous societies, women will have greater resource control and political activity when they have female allies; furthermore, that ecological factors influence women's access to such allies. We examined statistical associations among measures of ecological factors, the presence of female allies, and female power. The results of multiple regression analyses of ethnographic materials demonstrate that, cross culturally, ecological and sociocultural factors interact so that polygynously married women have more resource control and power when they are geographically close to their kin and have sisters as cowives. Additional statistical associations reveal how ecological factors moderate women's access to potential allies, which in turn are associated with resource control, female power/authority, and prevalence of negative attitudes about appropriate female behavior. (C) 2004 Elsevier Inc. All rights reserved.