The behavior of lactating female meadow voles ( Microtus pennsylvanicus ) and prairie voles (M. ochrogaster ) housed with partners of either sex was examined in an indoor, seminatural environment. Females were observed either alone, in the presence of the breeding male, or in the presence of a familiar, virgin female (sibling). Although there was no effect of partner condition on maternal behavior in either species, interactions between mothers and partners varied between species. Meadow-vole mothers spent less time in social interaction than prairie-vole mothers, and their interactions primarily were with female partners. In contrast, prairie-vole mothers primarily interacted with the male with whom they shared a nest and kept female siblings from entering the natal nest until pups were close to weaning age. The differential response of female prairie and meadow voles to partners of different sex is consistent with their winter social organizations.