The T cell receptor (TCR) σ locus lies within the TCR α locus and is excised from the chromosome by Vα-Jα rearrangement. We show here that σ sequences persist in a large fraction of the DNA from mature CD4⁺CD8⁻αβ⁺ mouse thymocytes. Virtually all σ loci in these cells are rearranged and present in extrachromosomal DNA. In immature αβ lineage thymocytes (CD3^−/loCD4⁺CD8⁺) and in CD4⁺CDS⁻αβ⁺ thymocytes expressing a transgene-encoded αβ receptor, rearranged σ genes are present both in chromosomal and extrachromosomal DNA. Thus, contrary to earlier proposals, commitment to the αβ lineage does not require recombinational silencing of the σ locus or its deletion by a site-specific mechanism prior to Vα-Jα rearrangement.