Social flexibility, that is, the expression of different types of social systems within one species, has been reported in several mammalian taxa, including rodents. However, sociality in rodents has been studied mostly in captivity and the results are often regarded as laboratory artifacts. We present field data for 2 populations of the striped mouse (Rhabdomys pumilio), a diurnal muroid rodent from southern Africa. The 2 populations inhabit different environments and demonstrate differences in social organization. R. pumilio in the arid succulent karoo lives in social groups, comprising multiple adults of both sexes that share 1 nest and the same territory. Striped mice in the moist grasslands of South Africa are solitary. Females inhabit exclusive territories and territories of males overlap those of several females; association between the sexes appears to be restricted to mating. Home ranges of females in the grasslands were 6 times larger and home ranges of males were 10 times larger than their counterparts in the succulent karoo. Onset of reproductive activity occurred earlier and at a significantly lighter body weight in grasslands. In the succulent karoo, sexually mature offspring remain in their natal group without reproducing. We suggest that group living in the succulent karoo is in response to habitat saturation and the benefits of philopatry, whereas living alone in the grasslands may be a response to limiting resources, such as food.