Mice deficient in either alpha beta or gamma delta T cells were more susceptible to infection with the protozoan parasite Cryptosporidium parvum than were control mice. Both neonatal and adult alpha beta-T-cell-deficient mice developed chronic infection. Gamma delta-T-cell-deficient neonatal mice were more susceptible than control mice but were able to clear the infection. Adult gamma delta-T-cell-deficient mice were not susceptible to infection. These data indicate that alpha beta T cells are important for resistance to C. parvum while gamma delta T cells have a less critical role.