Although the T cell y genes are similar in many respects to T cell receptor a and p genes, earlier studies suggested that only a single y variable (Vy) gene is expressed in mature T cells. We report the isolation and characterization of three new rearranged VT genes from murlne fetal thymocytes. Although each of the new Vy gene rearrangements is present in fetal thymocytes, two of them are undetectable In mature T cells. The levels of mRNA corresponding to each type of Vy gene rearrangement In mature T ceils am dramatically diminished compared with those in fetal thymocytes, although the abundance of two of the rearranged genes Is lncreasad in mature T cells. Our results demonstrate that there Is significant expressed variability of y genes in immature T cells. Furthermore, the dynamics of y gene rearrangement and expression support the Idea that y genes function In immature T cells.