Mammalian TCR delta genes are located in the midst of the TCR alpha gene locus. In the chicken, one large V delta gene family, two D delta gene segments, two J delta gene segments, and one C delta gene have been identified. The TCR delta genes were deleted on both alleles in alpha beta T cell lines, thereby indicating conservation of the combined TCR alpha delta locus in birds. V alpha and V delta gene segments were found to rearrange with one, both or neither of the D delta segments and either of the two J delta segments. Exonuclease activity, P-addition, and N-addition during VDJ delta rearrangement contributed to TCR delta repertoire diversification in the first embryonic wave of T cells. An unbiased V delta 1 repertoire was observed at all ages, but an acquired J delta 1 usage bias occurred in the TCR delta repertoire. The unrestricted combinatorial diversity of relatively complex TCR gamma and delta loci may contribute to the remarkable abundance of gamma delta T cells in this avian representative.