Variable/diversity/joining (V[D]J) recombination of the T cell receptor (TCR) and immunoglobulin (Ig) genes is regulated by chromatin accessibility of the target locus to the recombinase in a lineage- and stage-specific manner. Histone acetylation has recently been proposed as a molecular mechanism underlying the accessibility control. Here, we investigate the role for histone acetylation in the developmentally regulated rearrangements of the mouse TCR-gamma gene, wherein predominant rearrangement is switched from Vgamma3 to Vgamma2 gene during the fetal to adult thymocyte development. Our results indicate that histone acetylation correlates with accessibility, as histone acetylation at the fetal-type Vgamma3 gene in accord with germline transcription is relatively high in fetal thymocytes, but specifically becomes low in adult thymocytes within the entirely hyperacetylated locus. Furthermore, inhibition of histone deacetylation during the development of adult bone marrow-derived thymocytes by a specific histone deacetylase inhibitor, trichostatin A, leads to elevated histone acetylation, germline transcription, cleavage, and rearrangement of the Vgamma3 gene. These data demonstrate that histone acetylation functionally determines the chromatin accessibility for V(D)J recombination in vivo and that an epigenetic modification of chromatin plays a direct role in executing a developmental switch in cell fate determination.