Abstract. Sex-based-differences are known to affect susceptibility to protozoan infections, but their effects on parasitemia and clinical symptoms in Babesia infections remain unclear. We examined the sex-based susceptibility of various mouse strains to Babesia microti Munich strain infection. In all strains, male mice exhibited significantly higher peak parasitemia and more severe anemia than female mice. Testosterone and estradiol-17β treatment caused an increase in parasitemia and aggravation of anemia. Orchidectomized male mice receiving testosterone exhibited smaller splenic macrophage populations three days after infection, smaller B cell populations 10 days after infection, and reduced splenic tumor necrosis factor-α and interferon-γ mRNA expression than mice that did not receive testosterone. Mice receiving estradiol-17β did not exhibit immunosuppressive effects. Thus, a weakened and delayed innate immunity response may lead to acquired immunity failure. The results suggested that testosterone directly affects T or B cells, leading to delayed acquired immunity, dramatically increased parasitemia, and severe anemia.