The role of T lymphocyte subpopulations in the protection against intraperitoneal (i.p.) and peroral Encephalitozoon cuniculi infections was compared in adoptive-transfer experiments using severe combined immunodeficient mice. Whereas CD8 T cell–depleted, but not CD4 T cell–depleted, BALB/c splenocytes failed to protect the mice against i.p. infection, only SCID mice reconstituted with both CD4 T lymphocyte– and CD8 T lymphocyte–depleted splenocytes succumbed to peroral infection. The results indicate that whereas CD8 T cells are critical for the protection against an i.p. E. cuniculi infection, both CD4 and CD8 T lymphocyte subpopulations play a substantive protective role in a peroral infection, i.e., natural route of infection.