Molecular studies of the gamma delta TCR, which is expressed by a minor subpopulation 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 epithelia, which expresses a receptor of very limited diversity. In the developing murine thymus, gamma delta T cells appear in an ordered sequence of specific V rearrangements, V gamma 3V delta 1 on day 14, V gamma 2V delta 1 on day 17, and subsequently V gamma 4V delta 5, V delta 6, or V delta 7. We demonstrate that the transfer of expanded populations of gamma delta cells from newborn thymus and cell lines expressing the invariant V gamma 3V delta 1 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 V gamma 3V delta 1 T cells in thymic development in vivo is to promote CD4 and CD8 surface expression on precursor cells.