Microsporidia cause opportunistic infections in AIDS patients and commonly infect laboratory animals, as well. Euthymie C57B1/6 mice experimentally infected with intraperitoneal injections of lxlO6 Encephalitozoon cuniculi Levaditi, Nicolau et Schoen, 1923, Encephalitozoon hellem Didier et al., 1991, or Nosema comeum Shadduck et al., 1990 displayed no clinical signs of disease. Athymic mice, however, developed ascites and died 8-16 days after inoculation with N. comeum, 21-25 days after inoculation with E. cuniculi, and 34-37 days after inoculation with E. hellem. All athymic mice displayed hepatomegaly, dilated intestine and accumulation of ascites fluid. Granulomatous lesions were primarily located in the liver, lung, pancreas, spleen, and on serosal surfaces of abdominal organs.