Hematopoiesis originated by hematopoietic stem cells (HSCs) is distinguishable between fetal and adult mice. However, it is not clear whether the altered mode of differentiation is due to the change of properties of HSCs or different microenvironments in fetuses and adults. Here we show that fetal HSCs are fully capable of giving rise to all classes of B cells in the adult microenvironment. HSCs that are derived from fetal liver but not adult bone marrow (BM) of IL-7 receptor alpha chain (IL-7Ralpha)-deficient mice can also differentiate into B cells, suggesting that both IL-7 and thymic stromal-derived lymphopoietin (TSLP) are dispensable for fetal B cell development, because IL-7Ralpha is commonly used as a subunit of functional receptor complexes for IL-7 and TSLP. Similar IL-7/TSLP independent B cell potential is maintained by BM HSCs until 1 week after birth. In contrast, BM HSCs in mice older than 2 weeks of age absolutely requires IL-7Ralpha for B lymphopoiesis. These results demonstrate that fetal HSCs acquired adult characteristics between 1 and 2 weeks after birth in mouse BM.