The major pathway of gammadelta cell development is shown to be regulated by in-frame rearrangements at the T cell receptor (TCR) delta locus. Such "delta selection" occurs at or around the same point in thymocyte development as selection for in-frame rearrangements at the TCRbeta locus. However, there are at least two major differences with beta selection: first, delta selection commonly involves selection on the cognate TCR chain, gamma, suggesting that there is no "preTgamma" chain of major biological significance; second, most gammadelta-selected thymocytes differentiate rather than proliferate. Nonetheless, some delta selection events seemingly facilitate thymocyte expansion, similar to alphabeta T cell development. In these cases, TCRgamma selection is less obvious. Furthermore, the capacity of individual gamma chains to facilitate gammadelta selection is shown to vary with developmental age. The results further clarify early T cell development at the beta selection/delta selection stage and place clear constraints on models of cell fate determination.