Sam68 was identified as an SH3 and SH2 domain interacting protein for Src family kinases and is also a known substrate of Src kinases [39–42] and of the breast tumor kinase [43]. Sam68 has been shown to facilitate the export of unspliced HIV RNA [44] and to regulate pre-mRNA processing [45]. In the present paper, we report the generation of Sam68−/− mice and analysis of their skeletal phenotype. Our data indicate that the absence of Sam68 confers resistance to age-related bone loss in mice such that old Sam68 have a higher bone mass than their wild-type littermates. We provide evidence that Sam68 regulates the differentiation of bone marrow stromal cells by showing that cells isolated from Sam68−/− animals had enhanced osteogenic activity and decreased adipogenic activity than those harvested from wild-type littermates. Furthermore, Sam68−/− mouse embryo fibroblasts (MEFs) were impaired in their capacity to differentiate into adipocytes, consistent with Sam68 being a regulator of bone marrow mesenchymal cell differentiation. These results also characterize a new animal model to study bone metabolism, regeneration, and repair during aging.