Separation between primary and secondary lymphoid organs is a universal feature in jawed vertebrates. Strikingly, oncostatin M (OM)-transgenic mice present massive extrathymic T cell development, localized exclusively in the lymph nodes (LN). According to the prevailing paradigm, the thymus is the main source of T lymphocytes in gnathostomes mainly because thymic epithelial cells have a unique ability to support early steps in T cell development. It is therefore remarkable that productive T cell development occurs in the OM(+) LN, despite the absence of epithelial cells. The present study shows that in the OM(+) LN: 1) MHC class I expression strictly on hemopoietic cells is sufficient to support the development of a diversified repertoire of CD8 T cells; 2) the efficiency of positive selection of specific TCR-transgenic T cells is not the same as in the thymus; 3) negative selection is very effective, despite the lack of an organized thymic-like medulla. Furthermore, our data suggest that extrathymic T lymphocytes developing in the OM(+) LN undergo extensive postselection expansion because they live in the microenvironment in which they were positively selected. This work illustrates how the division of labor between primary and secondary lymphoid organs influences the repertoire and homeostasis of T lymphocytes.