Using gene knockout and wild-type C57BL/6 mice, we examined the role of alpha beta and gamma delta T cells in the resolution of Chlamydia trachomatis mouse pneumonitis (MoPn) biovar pulmonary infection. The results show that alpha beta T-cell-deficient (alpha-/-) mice, when compared with wild-type control mice, have dramatically increased mortality rate and greater in vivo growth of MoPn. The alpha beta T-cell-deficient mice were as susceptible to MoPn infection as T- and B-lymphocyte-deficient (RAG-1-/-) mice. Moreover, both alpha beta T-cell-deficient and RAG-1 mutant mice failed to mount delayed-type hypersensitivity (DTH) responses to organism-specific challenge and showed undetectable interferon-gamma (IFN-gamma) production by spleen cells upon in vitro organism-specific restimulation. In contrast, gamma delta T-cell-deficient mice exhibited intact DTH responses and their mortality rate and in vivo chlamydial growth were comparable to those in wild-type controls. More interestingly, gamma delta T-cell-deficient mice showed significantly higher levels of IFN-gamma production than did wild-type mice. The data indicate that the alpha beta T cell is the major T-cell population for acquired immunity to chlamydial infection and that gamma delta T cells may play an ancillary role in regulating the magnitude of alpha beta T-cell responses.