Chronic diarrhea is a common debilitating condition in AIDS patients. A new genus of the phylum Microspora Enterocytozoon bieneusi is the most frequently reported microsporidia in individuals with AIDS and is closely linked with chronic diarrhea and wasting. Although reported over 10 years ago, little is known about the infection and the disease it induces in humans due to a lack of in vitro and in vivo cultivation techniques. In an effort to establish an animal model we inoculated two SIV-infected rhesus macaques with duodenal aspirates from human AIDS patients with microsporidiosis. Both animals became infected with shedding of microsporidia in the feces beginning 7 and 9 days after infection. Light but consistent fecal excretion of spores, as confirmed by light microscopy and PCR, continued until they were euthanatized 7 and 9 months later, respectively, when they became wasted and severely ill. E. bieneusi, however, was judged to play no role, as the infection in the gut was extremely sparse. The failure to infect a normal, uninfected macaque with E. bieneusi highlighted the role of immunodeficiency of the host in this infection.