Unsporulated Isospora belli oocysts were detected in the stool specimens of three homosexual men. The oocysts were ellipsoidal measuring 23-33 X 12-15 micron. It is acid fast with modified cold kinyoun stain (MCK) and reveals orange fluorescence with the Truant's stain. Sheather's sucrose flotation method is effective in concentrating Isospora oocyst as with other coccidial oocysts. One specimen showed many Charcot-Leyden crystals. All three patients responded to treatment. Isosporiosis is rare in this country and is endemic in the tropics and subtropics. Although one case acquired the infection after returning from an endemic area, the other two cases had no travel history. This raised the suspicion that this, like other parasitic infections (amebiasis, giardiasis, cryptosporidiosis) may be sexually transmitted. Such exotic parasitic infections in homosexual men during the outbreak of the acquired immunodeficiency syndrome adds another unusual infectious agent to the differential diagnosis of diarrheal disease in this high risk group of population.