We searched for clonable committed T cell progenitors in the adult mouse bone marrow and isolated rare (≈0.05%) cells with the Thy-1^hiCD2⁻CD16⁺CD44^hiCD25⁻Lin⁻ phenotype. In vivo experiments showed that these cells were progenitors committed only to reconstituting the T cell lineage of irradiated Ly5 congenic hosts. Reconstitution of the thymus was minimal compared with that of the bone marrow, spleen, and lymph nodes. At limiting dilutions, donor T cell reconstitution of the spleen frequently occurred without detectable donor cells in the thymus. Progenitors were capable of rapidly reconstituting athymic hosts. In conclusion, the clonable bone marrow progenitors were capable of T cell reconstitution predominantly by means of an extrathymic pathway.