The information on the level of reproductive skew in rodent communal groups and the effect of polygyny on the female reproductive success is rather scant. The indices of reproductive success in mandarin vole females are compared in monogamous (MG) and polygynous (PG) groups under different laboratory conditions: in small cages with frequent handling of animals (30 MG, 15 PG) and in large glass tunnel systems without disturbance (20 MG, 16PG). Each PG consisted of two female sibs and an unrelated male; MG consisted of unrelated males and females. There was no reproductive skew within PG. On the contrary, the positive correlation between some indices of reproductive success was found in sisters. Independently of the maintenance conditions, both the proportion of females that gave birth within 45 days and the number of weaned offspring were greater in MG than in PG. Based on the data obtained in the small cages, the lower reproductive success in PG seems to result from some delay of breeding, reduced sizes of the first litters and, most important, increased mortality of newborn young. The behavior of voles living in tunnel systems was observed to quantify aggressive interactions and to reveal the causes of the pup mortality in PG. In general, the level of aggression was low in animals of both types of groups, but the rate of female-female conflicts in PG drastically increased during several days before and after the birth. Sisters exhibited permanent nest cohabitation and allonursing. Both nonmaternal and maternal infanticides resulted from social stress are suggested to be responsible for the high pup mortality in PG. Thus, in spite of the prevalence of PG in the free-living M. mandarinus population, there may be significant fitness costs associated with the communal breeding.