Microsporidia are unicellular organisms, which lack mitochondria. They have prokaryotic-like ribosomes and characteristic spores containing an extrusible polar tube which serves as a passage for inoculation of the infectious agent (sporoplasm) into host cells. Clinically apparent infections in man appear to be limited to immunoprivileged sites or immunocompromised patients. One species, Encephalitozoon cuniculi, has been reported several times in patients with neurological disorders and once causing a fatal hepatitis in an AIDS patient. The most recently discovered species, Enterocytozoon bieneusi, is known only from the small intestinal enterocytes of patients with the acquired immunodeficiency syndrome, and is easily differentiated from other microsporidia by the precocious development of spore organelles in the sporont and by the poor development of the endospore layer of the spore wall. Although only about 40 cases have been reported, circumstantial evidence suggests that E. bieneusi may be the cause of a severe watery diarrhoea, which responds only temporarily to treatment with metronidazole.