Isospora belli, an opportunistic protozoon, is one of the most commonly recognized causes of diarrhea in patients with acquired immunodeficiency syndrome (AIDS). Infection is acquired through fecal contaminated food or water, and generally diagnosed by examination of stool and/or duodenum biopsy specimens with acid-fast staining. Here, we present an uncommon case, chronic diarrhea caused by I. belli infection in a patient who is a renal transplant recipient.