Molecular studies of the γδ TCR, which is expressed by a minor subpopulatlon of T lymphocytes in all vertebrate species, have defined a subset which expresses a receptor with extreme junctional diversity and a second subset, most commonly found in eplthella, which expresses a receptor of very limited diversity. In the developing murine thymus, γδ T cells appear in an ordered sequence of specific v rearrangements, Vγ3Vδ, 1 on day 14, Vγ2Vγ1 on day 17, and subsequently Vγ4Vδ5, Vδ6, or Vδ7. We demonstrate that the transfer of expanded populations of γδ cells from newborn thymus and cell lines expressing the invariant Vγ3Vδ1 receptor into SCID mice, which lack T and B cells, results in the appearance of CD3−CD4+CD8+ thymocytes. Thus, one role of the early appearing Vγ3Vδ1 T cells in thymlc development in vivo is to promote CD4 and CO8 surface expression on precursor cells.