Two types of T lymphocytes, distinguishable by their surface expression of either the gamma delta or the alpha beta T cell receptor (TcR) for antigen, populate the periphery in the adult. In addition, immature precursors of both T cell types can be found in the thymus. While it is generally accepted that these two cell types represent distinct lineages, it is not known at which developmental stage these lineages diverge. The most mature thymocyte precursor population not yet expressing T lineage-specific surface markers (i.e. CD3, CD4, and CD8) is known to be capable of generating TcR-alpha beta T cells, and has been thought to be preprogrammed into the TcR-alpha beta lineage at an earlier developmental stage. We now show that this late-stage precursor is capable of giving rise to cells of both the TcR-alpha beta and -gamma delta lineages, both in vitro after intrathymic transplantation, and in vitro in simple culture medium or medium with cytokines. Thus it appears that the divergence of TcR-alpha beta and -gamma delta cells can occur at a relatively late stage of intrathymic development, just prior to the onset of CD4 and CD8 expression in most cells.