Mating systems and factors affecting reproductive success are much studied especially for mammals that are large bodied, have marked sexual size dimorphisms, and have a female-defense mating system. For species that deviate from these patterns, we need more information on mating systems and reproductive success. Here, we study mating system and factors related to reproductive success in a solitary rodent, the Siberian flying squirrel (Pteromys volans). In contrast to most other mammals, males are not larger than females in flying squirrels. Similarly to most mammalian species, we observed multimale paternity within litters and reproductive success of males being positively related to body mass. Variation in reproductive success was clearly higher for males than for females, although remained lower than observed in species with highly male-biased sexual size dimorphism. Female flying squirrels lived in nonoverlapping home ranges and reproductive success was positively related to body mass, in line with earlier predictions for large female size in mammals.