Microsporidia are a group of obligate intracellular protistan parasites that have recently gained recognition as opportunistic infections in persons with AIDS or in those who are otherwise immunocompromised. To date, eight genera have been identified as human pathogens, Encephalitozoon, Enterocytozoon, Nosema, Vittaforma, Pleistophora, Trachipleistophora, Brachiola, and the collective genus Microsporidium. A frequent causative agent of microsporidial diarrhea and systemic infection among AIDS patients is Encephalitozoon intestinalis. This parasite can also cause chronic diarrhea in immunocompetent persons. However, many epidemiologic aspects of this infection remain poorly understood as the diagnostic tools for this parasite are not amenable to widespread use. In an attempt to improve this situation, we have developed an ELISA test to detect human serum antibodies reactive with Encephalitozoon intestinalis.