Superoxide dismutases (SODs) play an important role in protection against oxidative stress and have been shown to contribute to the pathogenicity of many bacterial species. To determine the function of the mycobacterial copper and zinc-cofactored SOD (CuZnSOD), we constructed and characterized Mycobacterium tuberculosis and Mycobacterium bovis BCG CuZnSOD null mutants. Both strains were more sensitive to superoxides and hydrogen peroxide than were their respective parental strains. The survival of M. bovis BCG in unstimulated as well as activated mouse bone marrow-derived macrophages was not affected by the loss of CuZnSOD. The survival of CuZnSOD deficient-M. tuberculosis in guinea pig tissues was comparable to that of its parental strain. These results indicate that the mycobacterial CuZnSOD is not essential for intracellular growth within macrophages and does not detectably contribute to the pathogenicity of M. tuberculosis.