The V gamma 2 V delta 2 T cell subset responds to Bacille Calmette-Guerin (BCG) immunization in macaques and may be a component of protective immunity against tuberculosis. We characterized the effects of BCG on the V gamma 2 V delta 2 T cell receptor repertoire by comparing the starting population of V gamma 2 chains in cynomolgus macaques with the repertoire found after priming or booster immunization with BCG. The starting repertoire was dominated by public V gamma 2 chain sequences that were found repeatedly among unrelated animals. Primary exposure to BCG triggered expansion of cells expressing public V gamma 2 chains and booster immunization was often associated with contraction of these same subsets. Thus, BCG-reactive V gamma 2 chains were present at high frequency in the repertoire of mycobacteria-naïve macaques and they comprised the major response to primary or booster immunization. Normal selection processes that created the naïve V gamma 2 repertoire in macaques, also encoded the capacity for rapid responses to mycobacteria. The unusual composition of a normal V gamma 2 repertoire helps to explain the powerful gammadelta T cell responses to BCG immunization.