In peripheral lymphoid tissues of TCR transgenic mice that express the nominal antigen (HY peptide plus H-2Db MHC) recognized by the transgenic TCR, there exist unusual CD4-CD8- and CD4-CD8low cells bearing the transgenic TCR. Here we show that, unlike TCR alpha beta T cells that are generated in the absence of nominal antigen, these unusual cells do not express endogenous TCR alpha genes, have maintained the TCR delta locus on both chromosomes, and can coexpress TCR alpha beta and TCR gamma delta chains on the cell surface. The latter is also true for CD4-CD8-, HSA+ TCR alpha beta + thymocytes in male and female TCR transgenic mice. The number of TCR alpha beta and TCR gamma delta coexpressing cells is increased in pre-TCR-deficient mice. The data indicate that the TCR alpha beta can replace the TCR gamma delta in the development of gamma delta lineage cells and that the pre-TCR interferes with the generation of gamma delta-expressing cells.