In the last 4 years, we have identified an acid-fast, autofluorescent organism in the stool of seven patients with diarrhea. The organism was identified as a cyanobacterium-like organism by the Centers for Disease Control (Atlanta) and as a coccidian by researchers in Peru and at the University of Arizona (Tucson). We present reports on the seven cases and a review of the literature. Three patients were known to be seropositive for the human immunodeficiency virus (HIV). All complained of watery diarrhea that had begun 3 days to 8 months before presentation. Colonoscopy in two patients showed a normal mucosal pattern; a biopsy for one of these patients showed chronic nonspecific inflammation of the colon. Examination of stool for ova and parasites revealed multiple oval and round nonrefractile organisms with well-defined walls that resembled large cryptosporidia; they measured 8-9 microns in diameter. These organisms did not stain by Giemsa or gram methods but were acid-fast by Kinyoun carbolfuchsin and Ziehl-Neelsen stains. The organisms fluoresced as blue under ultraviolet light. In the immunocompetent patients, diarrhea lasted an average of 19 days and resolved spontaneously. Diarrhea persisted in the HIV-seropositive patients. The clinical course and organism resembled those reported for travelers and HIV-seropositive patients. This organism may represent a newly identified cause of watery diarrhea in humans.